What are the most common monster weaknesses?

[Katzekerl]

My next character is a monster slayer ranger, mostly for flavor reasons. I would like to be able to take advantage of the hunter’s sense by preparing ways to take advantage of common weaknesses. Since I don’t own the monster manual or have extensive experience with this game, I turn here for collective wisdom.

[JackPhoenix]

Depends on campaign, obviously, but the most common vulnerability amongst creatures in MM, VGtM and MToF is fire (but it's also 2nd most resisted after poison, and just before cold (more creatures are immune to poison than fire, and more creatures are resistant to cold than fire), followed by bludgeoning (mostly when skeletons are concerned). And that's about it. Fire (along with radiant damage) screws few more creatures due to blocking regeneration.

Radiant and force are the best damage types in the game, due to only one creature being immune to both. Few creatures (angels) are resistant to radiant, but there are other creatures with weakness against radiant damage... it stops vampiric regeneration and Undying Fortitude of zombies

Very few creatures have vulnerabilities in 5e.

[MaxWilson]

My next character is a monster slayer ranger, mostly for flavor reasons. I would like to be able to take advantage of the hunter’s sense by preparing ways to take advantage of common weaknesses. Since I don’t own the monster manual or have extensive experience with this game, I turn here for collective wisdom.

By far the most common monster weakness is ranged combat. Even those few monsters that have ranged attacks (like giants) tend to have even stronger melee attacks (about twice as strong in the case of giants). And unlike PCs, monsters won't typically be able to exploit partial cover (because PCs can specialize in feats that let them simply ignore it) or high mobility.

Even if you do something as simple as just take the Mobile feat as a variant human, learn Expeditious Retreat at level 1, and fight using ranged cantrips when you can, you'll trivialize probably 40%-70% of the enemies you encounter in Tier 1, depending on your DM's style, even if they would otherwise be Deadly fights.

Monsters also tend to be weak against Athletics, so if you want to specializing in grappling/etc. you can. Here's an amusing recent thread that may open your eyes to some potential tricks: https://forums.giantitp.com/showthread.php?624030-The-Hextuple-Psychic-Piledriver-Or-how-to-wield-THE-PLANET-as-a-20d6-weapon

[Unoriginal]

My next character is a monster slayer ranger, mostly for flavor reasons. I would like to be able to take advantage of the hunter’s sense by preparing ways to take advantage of common weaknesses. Since I don’t own the monster manual or have extensive experience with this game, I turn here for collective wisdom.

Resistance or immunity to weapon damage is generally bypassed by magic weapons. Silvered weapons work against all the types of lycanthropes, the jackalweres, and the Devils. Adamantine weapons work against golems and some other constructs IIRC.

Most creatures are not immune/resistant against fall damage or against any damage not from a weapon. The Unarmed fighting style, for example, let you do 1d4 damage to a creature you're grappling, bypassing immunity/resistance against non-magical weapons.

[Rusvul]

If you're using ranged weapons, it should be pretty easy to pick up silvered and adamantine arrows to deal with fiends/lycanthropes and constructs that might resist weapons not made of those materials. (In prior editions, fey resisted weapons not made of cold iron. If your DM uses the third-party Tome of Foes book--unlikely--then it might be worth picking them up, but otherwise they don't exist in 5e.)

Additionally, Alchemist's Fire is useful for dealing with monsters that regenerate unless they take fire damage--most notably trolls, but also some plant creatures iirc--especially at low levels or if you don't have a party member who can consistently deal fire damage. (Acid is better for dealing damage, but Alchemist's fire's damage-over-time effect can stop regeneration for multiple rounds.)

Holy water can be useful for some undead creatures--like with Alchemist's Fire, it's best at low levels or if nobody else can deal radiant damage.

Not monster-specific, but caltrops, hunting traps, and marbles are all decent at low-level if you have a few rounds to set up.

Other than that, MaxWilson is absolutely correct, ranged attacks are highly effective against many, many monsters--once again, especially at low levels. The nature of 5e is that relatively few monsters have weaknesses of any kind to capitalize on, and those that do are often irrelevant by the later levels--usually because magic weapons bypass their resistances just as well as specialized weapons do, or because any kind of non-magical preparations you can make become untenable due to low DCs, low damage, or poor action economy.

If you really want to feel prepared for anything, you should--if your DM allows it--consider stocking up on/scribing scrolls for spells that have very niche use cases (possibly by taking a level of a class like Wizard or Cleric that has more situational/utility spells, as well as opening up rituals). There's something really fun about being able to pull a scroll of Fog Cloud out of your back pocket when you really need some cover, or Heroism when fighting a monster with fear effects. There's lots of spells that aren't worth preparing most of the time because they're so niche, but are very good to have when they're good--and I think that scribing scrolls is a very useful and fun way to prepare for those kinds of situations. It's not the most ranger-friendly, though, more of a spellcaster tactic--still, there's a handful of niche-but-useful spells on the ranger list. Maybe you could learn some, scribe a bunch of scrolls, and then swap them out when you level up? Or find someone else to scribe them for you.

[Aett_Thorn]

Cupcakes.

Even the most hardened evildoer can be tempted by a good cupcake.

[Vogie]

What you are looking for, sadly, doesn't really exist in the core game.

There are GMs out there who either are or want to play games similar to what you are talking about - "Witcher" or "Supernatural" style games, where the monsters with lots of immunities and a handful of exploitable vulnerabilities. Sadly, outside of the occasional ones that everyone knows - werewolves, vampires, golems - there isn't much to exploit.

Now, if you happen to be lucky enough to BE in one of such games, where you need to know that your target monster needs to be either defeated with, I don't know, silvered weapons, or slain while under the light of the moon... GREAT! You picked the perfect subclass for it. Also, I'm jealous.