The Great Devourer
Synapse - Synapse is a defining trait of the Tyranid faction - if you don't like it and aren't prepared to deal with it, then you shouldn't be playing Tyranids. How much Synapse should you take in an army? As a general rule of thumb, think of them like you would think of Troops/Scoring choices; these should be "[Your Army's Points Cost divided by 4]+1" - So in 2000 points you should have at least 5 Troops/Scoring units. There's a little bit of room to negotiate - you can probably get away with 4, if you have durable 30-strong units of 'Gaunts with FNP and the likes - but not very often. Treat Synapse in a similar vein - for safety, roughly one in every four of your units should be Synapse, or failing that, immune to Instinctive Behaviour. Be prepared to tinker specifics, depending on results.
It is possible to build an entire army that ignores Synapse - it's quite easy to play with, but very hard to win with. Again, if you don't want to play with Synapse, don't use Tyranids. At all. Seriously.
Instinctive Behaviour -Should never come up, if you've built your army well and don't play like a fool. Having said that, you can manipulate some units with the (Feed) and (Hunt) tables so that failing I.B. is not only not a problem, but actively useful - your Haruspex is probably going to "Devour" anyway, so why wouldn't you want it to roll and try to get "Kill" as well?
Shadow In The Warp -Enemy Psykers within 12" have -3 to their Leadership. This was far, far more useful in 6th Edition when Leadership dictated how successful you were at casting Psychic Powers, but now all it does is make Perils of the Warp rolls more dangerous. Remember though that it's a blanket Leadership modifier and thus also affects Morale Tests - in a meta where Psykers are also likely to be your opponents' Warlord, it can be handy to plough a Flying Hive Tyrant into a Psyker and their bodyguard, reap a couple of casualties and rout an expensive and/or key unit quickly.
Hive Tyrant - Your best unit by quite along shot. Wings are mandatory, but there's a lot of freedom in your wargear depending on what role you want it to play. Twin Devourers makes for a good anti-flyer creature; Lash Whip, Bonesword, Toxin Sacs and Adrenal Glands makes it a close combat monster; Lash Whip & Bonesword and then a decent gun, either a Heavy Venom Cannon or the Miasma Cannon depending on what you're targetting, is a reasonable balanced unit. You don't really need much more than that, but if you are considering a Tail Biomorph, remember that ALL close combat attacks made by a Monstrous Creature are AP2. Regeneration is expensive, but it's big and clever on any Warlord unit that doesn't have a 2+ Save or Eternal Warrior (ie: All of them).
Tyrant Guard Brood - If your Tyrant doesn't have wings, you're taking some of these guys; there's really no way around it. A couple, unarmed, make for a perfectly good ablative shield, but Boneswords & Lash Whips (though pricey) makes them incredibly deadly.
Tervigon - A very popular choice, though most often as a Troops choice. Big piles of Termagaunts are a good, solid way to build your army, so why not have more of them and make them better along the way? You don't really need many upgrades as you're mostly there to support your 'Gaunts, but there's a good argument that Regeneration can be worthwhile if you're worried about it being blown up and taking all of its broods with it. Remember that Tervigons are not Characters, and even though they can be your single HQ choice they cannot get a Warlord Trait.
Tyranid Prime - Your bargain-basement HQ choice.... By which I mean, it's only slightly cheaper than everything else. Still, he buffs your Tyranid Warriors (who are good candidates to be carrying Bio-cannons) and can carry a Miasma Cannon himself as cheap (compared to a Tyrannofex or Hive Guard) fire support.
Tyranids don't get any of the MRB Psychic Disciplines.
This is a Bad Thing.
PRIMARIS: Dominion - Doesn't even work in 7th. Instinctive Behaviour is tested for at the start of the turn and Blessings are cast in the Psychic Phase, which comes after the Movement Phase, which is after the start of the turn.
Catalyst - Feel No Pain on the Psyker and another nearby unit is great, be it for a Hive Tyrant trudging his Hive Guard up the flank or a Tervigon drowning your opponent in a sea of 'Gaunts.
The Horror - Potentially useful if you like using units like Lictors or infiltrating Genestealers, so that whatever they appear next to won't immediately blow them away, but otherwise weak.
Onslaught - Yes, Hive Tyrant, you can target yourself. Or on your biggest unit of 'Gaunts - they're fodder anyway, but that extra round of shooting can mean a LOT of firepower being thrown at your opponent.
Paroxysm - Feeble due to RNG, but it's the closest that you can get to Invisibility when your FMC lands in front of an enemy unit.
Psychic Scream - Reasonable on a Winged Tyrant who can get close enough to use it and then capitalize with an Assault, but no one else cares much for it.
Warp Blast - The Lance is a handy anti-tank shot in an army that can sometimes struggle to find such things, and the Burst is one of your few AP3 shooting attacks. If you get this, you shouldn't be too disappointed, particularly on a Winged Tyrant who can also pick up a buff for himself.
1: Nature's Bane - At the start of your movement phase, choose a piece of terrain within 12" of your Warlord; it becomes Carnivorous Jungle until the end of the game. Put it on something with Wings/Outflanking/Infiltrate and get it into your opponent's Deployment Zone and on his objectives as quick as possible - it's not massively powerful but it is hilarious.
2: Heightened Senses - Night Vision in a 12" bubble. Tyranid Primes quite like it, but everyone else can do better.
3: Synaptic Lynchpin - Synapse range is 18". Is what it is; possibly wasted on a Flying Tyrant, since he can get out of 18" in a turn if he really wants to. And probably will.
4: Mind Eater - Killing an Independent Character in Assault gives you an extra Victory Point. Awesome on a Tyrant decked for Assault. He will reap almost anything put in front of him, so challenges are even more attractive. Likely to be wasted on everyone else.
5: Digestive Denial - Nominate a piece of terrain in your opponent's Deployment Zone; the cover save it provides is reduced by 1 (minimum 6+). On first glance not great - most of your weapons have a high AP so most enemies will be getting their normal saves most of the time. Against lightly armoured enemies that NEED a cover save, though, it can be very good.
6: Adaptive Biology - After taking a wound, the model gains Feel No Pain (5+). Tervigons like it because everything and their dog will be shooting at them; Hive Tyrants like it because they will likely be up in someone's face getting their hands dirty; Primes are likely to get squished before it comes into effect. FNP is never a bad thing, it's just a shame that you need to get hurt before it kicks in.
Activating this ability requires the model to have taken a wound, but not necessarily to be MISSING a wound; if they have the Regenerate biomorph, they don't lose FNP when they get that wound back.
Your Traits are telling you that you ought to have a Monstrous Creature (preferably a Hive Tyrant) as your Warlord; two of them are good for those guys, two screw with your opponents' terrain and the other two kinda go either way. Very little is outright trash, which is nice.
The Maw-Claws of Thyrax - The only way to give Rending to a Tervigon (which is a Monstrous Creature, and so always hits at AP2 anyway...), otherwise it gives Preferred Enemy with conditions. Cheap but not great.
The Miasma Cannon - A 'Blast' and 'Template' weapon that's also Poison (2+). Good for busting cover, but it's on the expensive side and is terrible against non-Infantry units, the latter usually being in high demand.
The Norn Crown - Add 6" to your Synapse Range. Sounds okay, but for 40pts!? That's half of a Tyranid Warrior Brood, or most of a Zoanthrope Brood, or the difference between a Trygon and Trygon Prime which not only becomes Synapse but gets more stuff as well. No errata to say otherwise, so it presumably stacks with Synaptic Lynchpin warlord trait and the Dominion Psychic power for a maximum range of 30".
The Ymgarl Factor - Expensive, and you have to think 2 turns ahead when planning which bonus you want and when, which can easily be disrupted by particularly cunning tactics.... Like, say..... your opponent shooting you. Or declaring an assault. You're generally better off getting the same bonuses from other sources (a bodyguard, or an additional weapon) and having them on all the time.
The Reaper of Obliterax - Very expensive, with lots of special rules, and it feels like there's always something going to waste. Monstrous Creatures already strike at AP2, so Rending is kind of unnecessary, and Lash Whips or Bone Swords are a lot cheaper if you need Swiftstrike and/or Instant Death. 45pts for Shred? Poison only costs you 10pts, and you're likely to get a reroll from that.
All in all, nothing jumps out and screams "Take me! Take me!", and a lot that can be taken more cheaply through other upgrades.
Tyranid Warrior Brood - Synapse and Scoring is a good combination, but it makes them prime targets for your opponent and all they have is 4+. Take a couple of squads, give them long ranged weapons and then hide them on an objective where they can make full use of a cover save, particularly if your HQ is a Winged Tyrant who is off doing his own thing and likely out of Synapse range. The Prime is a worthwhile upgrade for his stats boost, but he's still very fragile.
Genestealer Brood - Quickly gets expensive, but they have a long list of cool special rules. A unit of 10 with a Broodlord will Infiltrate up to your opponents' objective and remove pretty much ANYTHING from it - one of the few reasons to take Lictors, even if you subsequently just use them as a screen for the turn before the Genestealers can assault. In fact, the Broodlord is so good in challenges and The Horror can potentially save your from a round of shooting, so you should take him anyway. If you take Adrenal Glands and Toxin Sacs you get to reroll for the purposes of Rending, but now you're paying 20pts per model; for something that's inevitably going to get Overwatch'd, and Overwatch'd a LOT, I think that's too much. Send the Lictor in first to soak up fire, maybe....
Termagaunt Brood - Cheap and able to put out a ridiculous number of shots when (not 'if') you equip them with Devourers. Just keep throwing lots of dice at your opponents' 3+ and 2+ saves until they start to fail them, and save your high strength/low AP weapons for vehicles.
You can take a unit of 30 or more to also take a Tervigon as a Troop instead of HQ. Do this. Do it lots.
Hormagaunt Brood - If all you need are big, cheap piles of troops then Termagaunts are generally better as they can shoot and be replenished by Tervigons. If you need a fast shock troop on the cheap, a Hormagaunt with 'Glands and 'Sacs is half the cost of a Genestealer. I like them, but in the current meta I wouldn't recommend that you takes LOTS of them.
Ripper Swarm Brood - They're very cheap, and they're now Scoring. But then, so are Termagaunts who have guns, don't take extra damage from Blasts, and get buffs from Tervigons. Pretty redundant, really.
Hive Guard Brood - s8, Ignores Cover, T6 and W2 - they're great. Always take a unit of them to deal with Transports and other tough guys. If you aren't taking a unit if them, it's because you're taking TWO units of them.
Lictor Brood - As a unit, they're not very good. They're not very tough, they're not great in combat, and their saving throw sucks - they're Flamer/Bolter bait. What they're very good for is Infiltrate and Pheromone Trail, to get a much better unit in position quickly. You will never need more than one brood, and probably not even 3 models in it - stick 'em somewhere near your opponent, but in cover, and hide. Forever.
Zoanthrope Brood - They've taken a hit in their new edition, but an Assault 3 Warp Lance is even better at killing vehicles than Hive Guard, for about the same price. Wrap them in cotton wool if you want them, aim everything they have at the biggest target, and don't be surprised when they go splat - 3++ is only as good as 3+, when Sternguard are Rapid Firing you into oblivion.
They're also a repeatable unit that generates Deny The Witch dice at a reasonable rate - add both of these things together and it's usually worth taking a unit even if keeping them alive requires a lot of micromanagement.
Venomthrope Brood - There's a lot of competition for good Elite choices in this book, and Venomthropes are one of those units that you think you ought to take but everything else seems.... better. They're a little too expensive and they're only T4 5+ so they WILL die. It's better to remember that units like Termagaunts don't need a Cover Save, they provide one for the units that matter. If you want one, fine; sit it next to a unit of 30 Termagaunts and Go To Ground on an objective. But really, you're not making the best use of a slot.
Pyrovore Brood - Ugh. Niche, even when you could 'Drop Pod' them into play. Now they're still weak and wretched. Avoid.
Haruspex - A Monstrous Creature that wants to get into Assault, and is actually quite good at it - neat! Honestly I'd prefer to take a second Hive Tyrant instead, for the same role, but if you're strapped for points he's a good alternative.
Tyranid Shrike Brood - Tyranid Warriors with wings, otherwise with the same upgrades. They're even more fragile, losing their 5+ saves against Boltguns, so don't need them to get close to the enemy. They also aren't scoring so you're far better off taking ordinary Warriors that do.
Ravener Brood - Same as with Genestealers - they can't assault on the turn they arrive, which is what they really WANT to do, and they're quite expensive. They can make up for the former by taking some ranged weaponry, Deep Striking further away from the enemy than you might expect and then making up the distance by being Beasts. Overall, in a very similar role Genestealers have the edge as you don't have to rely on random Deep Striking rolls and 'Stealers are cheaper, despite both units dying to the same sort of opposition.
The Red Terror -He doesn't work on a lot of things that you'd ideally like to get rid of quickly - Monstrous Creatures, guys on bikes, guys in Terminator Armour and the likes - but his stat line is pretty good and when it works his Swallow Whole ability can be handy. Perhaps a little too expensive, as are Ravenors in general, for you to rely on heavily, but he's fun. Also note that he's one of the few Characters in the Codex (along with a Broodlord), so he can sometimes count as your Warlord if you don't have a Character in a HQ slot. Note that the Red Terror is not an upgrade to the existing unit - he's "added" to it, and thus is his own model in addition to the normal Ravenors.
Sky-Slasher Swarm Brood - Ripper Swarm Broods have only two good things; they are cheap, and are scoring. Sky-Slashers are the same, but do not score. Avoid.
Gargoyle Brood - Cheap. Fast. Decent amount of firepower. Definitely a good 'screen' for a flying HQ choice. Don't take their upgrades, they don't want to be in Assault - just take more Gargoyles instead.
Harpy - Flying Monstrous Creatures are generally good anyway. This one is the cheaper of the two, and highly prefers fighting Infantry. With their Spore Mines they'll happily take out a blob, but they don't really have the stats or equipment to go toe-to-toe with something elite by themselves. Send in the Tyrant instead.
Hive Crone - High strength Vector Strike and a Template weapon is a good match - get in there and use 'em. Tentaclids are there for dogfighting flying vehicles, but they're still not great - you're better off taking a Quadgun and manning it with some Warriors. Priced about right, but - apart from it's melee attack - you have no really reliable way of dealing with tough vehicles, so bring two if you can.
Spore Mine Cluster - A really cheap source of AP4 that doesn't give away Victory Points. Between your hordes of Termagaunts, Harpies and the likes you shouldn't be struggling to hurt blobs of enemy infantry, but AP4 is important to you. Don't go out of your way to take lots of them, but if you have a few spare points and don't want to have to micromanage a minimum sized unit of 'Gaunts or Rippers, do remember these guys.
Carnifex Brood - Carnifex Broods are the sort of unit that you have to build your army around - not necessarily because they're good (they can be), but because they're such a big investment. Never take just 1 in a Brood; that's just an easy target for missile launchers. Don't go crazy with the upgrades - naked is fine, Adrenal Glands to make them Fleet is good, Toxin Sacs for rerolls, maybe if you're feeling unlucky, but anything else is a luxury you can probably do without. As for ranged weapons, Spine Banks can be handy as a way to thin out a blob and prevent tarpits, but you certainly don't need to give everyone Bioplasma.
Biovore Brood - AP4 Artillery for a reasonable price. These are preferable to the Spore Mine Clusters on their own, as even a little bit of accuracy can be a big improvement.
Trygon - Subterranean Assault is basically a poor version of Deep Striking under an Icon or Teleport Homer, and as such comes with a list of problems that you should know about ahead of time. Long story short; it's not brilliant, but it has it's uses. Decent in a fight, works well in pairs or with a Mawloc as back up, but they're too expensive to enjoy all the time.
Trygon Prime - When Shadow in the Warp worked properly, Primes were almost an automatic upgrade. Now, however, you're paying a lot of extra points for +2Ld on a Fearless creature and an upgrade to a so-so shooting attack. In 7th Edition, you pay a lot of points on top of an already expensive Trygon for almost no return. Don't bother.
Mawloc - If your Reserves rolls are hot, it's a really, really good way to disrupt an entrenched enemy unit early in the game. Not fantastic in a stand-up fight, but you should never be in one. Hit, Run, Burrow, Repeat. Don't forget that if you're off the table when the game ends, even if you did it voluntarily, then a lot of scenarios will count you as a casualty for VP purposes.
Exocrine - A rare source of AP2 shooting, but it's range is kinda short. If your meta is rich with Terminators then you'll want a couple, but otherwise it's hard to recommend such a big target when Hive Guard are better at killing vehicles and you have plenty of ways to kill non-2+ infantry.
Tyrannofex - The Rupture Cannon is very important - it's one of only three ranged weapons in the book that is above Strength 7, and it's Assault 2 to boot. You should be taking one of these if there's even the remotest chance that you're opponent has AV12+ vehicles. (Protip: They almost certainly are.)
The Swarmlord - His biggest drawback is that he doesn't have wings - a winged Hive Tyrant is a pretty big deal - but apart from that, he's awesome. He boosts the rest of your army, he's great in challenges, and he's not too shabby at killing other stuff too. He will need a bodyguard, however, because a lone Monstrous Creature is a big and inviting target so be prepared to spend even more points on some friends for him.
Old One Eye - Hits like a truck, but he's I2 without an Invulnerable Save so he doesn't want to be in any 'serious' challenges, and since he's not an Independent Character he can't have a bodyguard and so will probably get shot to pieces before he gets the chance. He's a very expensive fire magnet, even more so than an ordinary Carnifex, for little extra in return.
Deathleaper - He's more survivable than an ordinary Lictor because he can only be fired at by Snap Shots, but he's still only T4 and 5+ so one lucky Missile/Melta and he's toast. To say that he costs nearly as much as three Lictors, for what he does you're much better off taking three Lictors instead.
Possibly due to an oversight by the work experience intern who wrote the errata, Tyranids CAN currently make use of fortifications.
You need a Quad Gun - there's no denying it. Sadly your best candidate to use it is a BS4 Tyranid Prime, but the alternative is a BS0 Broodlord..... Yeah.
Lots of criticisms have been leveled against Come The Apocalypse allies, but quite frankly it's a huge improvement as far as Tyranids are concerned, as they can now have SOME kind of ally AT ALL.
And to be frank, it's not even that big of a deal - most Tyranids want to run up close and either shoot short-ranged weapons or be in Assault, and what they NEED in an ally is the ability to inflict high strength anti-vehicle attacks at long range, so it's quite possible that your two forces will never be close enough to worry about One Eye Open or the rest of it.
In this, look for units such as Tau Battlesuits, or - if you're particularly interested in Forging a (plausible) Narrative - Astra Militarum for their tanks, artillery and heavy weapons. Genestealer Cults are still a thing in the fluff, after all.
You also want to keep an eye out for anti-flyer units - again, Tau and Astra Militarum are some of your best friends.