How would you level a small beastmaster that rides his/her companion?

[Spacehamster]

So seen people mention that a halfling or gnome for that matter BM that rides his pet into battle can be pretty formidable but what would be the best way to optimize this? As in do you go single class BM or do you MC, what feats, what weapons and so on. :)

[Giant2005]

The only combat feat you want is Mounted Combatant.
You want to strive for 11 levels of Ranger and 3 of Cleric asap. If you need more feats/ASIs then take Ranger to 12 and Cleric to 4. After that, either load up on more Cleric levels for the sake of more spell slots, or if your stats allow for it, take Sorcerer levels so you can quicken spells and cast Shield as a reaction. If you have a DM that is willing to call it an attack action when you have your companion attack, then 3 levels of Battlemaster would be ideal for the sake of Commander's Strike (you don't want to attack at all yourself if you can handle it. The ability you care the most about is Wisdom, and the only fighting style that matters is the defense fighting style.
Key features that you will gain at specific class levels:
Ranger 1: Absorb Elements. This will prevent you from taking tremendous AOE damage when you and your companion are hit by an AOE while under the effect of Warding Bond.
Ranger 1: Beast Bond. Your best use of concentration until you reach Ranger level 9.
Ranger 9: Conjure Animals. This is a great way to spend your Concentration that will increase your DPR a lot without breaking Sanctuary.
Ranger 11: At this level your companion (A giant snake - do not choose anything but a giant poisonous snake) will out-damage most of the other characters.
Cleric 1: Sanctuary. This spell turns you into one hell of a tank. Your companion can't be targeted by attacks and this spell will make your enemies struggle to target you.
Cleric 1: Heavy Armor proficiency. Your AC matters whereas your Str and Dex do not. Choose a Cleric subclass that gives you Heavy Armor proficiency and don't even worry about the strength requirement if you don't want to - that requirement only matters when you aren't mounted, and you will never be unmounted if you can help it.
Cleric 3: Spiritual Weapon. This gives you something to do with your bonus action that doesn't come at the expense of concentration (which is better spent on Conjure Animals or Blur).
Cleric 3: Warding Bond. This will halve the damage taken by your companion against any attacks that can actually harm it (anything that isn't an attack roll - most notably AOE damage).
Sorcerer 1: The Shield Spell. This is just one more layer to your defenses.
Sorcerer 3: Blur. Again, another layer of defenses - this time coming at the expense of concentration (so you must decide if you want more offense from Conjure Animals, or more defense from Blur).
Sorcerer 3: Quicken Spell. This gives you a more expensive, but more powerful use of your bonus action than Spiritual Weapon. It is an asset in difficult fights.
Battlemaster 3 + Ranger 5 (and a permissive DM): This will give you something to do with your leftover attack when you have your companion attack.

Prioritize any of the above abilities in whatever order you like, depending on what abilities you value more. The highest priority are the Cleric and Ranger levels. With just the Cleric and Ranger levels, you become a DPR machine that can out-tank anyone else on the field under most conditions. The Sorcerer levels just help you excel even further in both aspects.

[Spacehamster]

The only combat feat you want is Mounted Combatant.
You want to strive for 11 levels of Ranger and 3 of Cleric asap. If you need more feats/ASIs then take Ranger to 12 and Cleric to 4. After that, either load up on more Cleric levels for the sake of more spell slots, or if your stats allow for it, take Sorcerer levels so you can quicken spells and cast Shield as a reaction. If you have a DM that is willing to call it an attack action when you have your companion attack, then 3 levels of Battlemaster would be ideal for the sake of Commander's Strike (you don't want to attack at all yourself if you can handle it. The ability you care the most about is Wisdom, and the only fighting style that matters is the defense fighting style.
Key features that you will gain at specific class levels:
Ranger 1: Absorb Elements. This will prevent you from taking tremendous AOE damage when you and your companion are hit by an AOE while under the effect of Warding Bond.
Ranger 1: Beast Bond. Your best use of concentration until you reach Ranger level 9.
Ranger 9: Conjure Animals. This is a great way to spend your Concentration that will increase your DPR a lot without breaking Sanctuary.
Ranger 11: At this level your companion (A giant snake - do not choose anything but a giant poisonous snake) will out-damage most of the other characters.
Cleric 1: Sanctuary. This spell turns you into one hell of a tank. Your companion can't be targeted by attacks and this spell will make your enemies struggle to target you.
Cleric 1: Heavy Armor proficiency. Your AC matters whereas your Str and Dex do not. Choose a Cleric subclass that gives you Heavy Armor proficiency and don't even worry about the strength requirement if you don't want to - that requirement only matters when you aren't mounted, and you will never be unmounted if you can help it.
Cleric 3: Spiritual Weapon. This gives you something to do with your bonus action that doesn't come at the expense of concentration (which is better spent on Conjure Animals or Blur).
Cleric 3: Warding Bond. This will halve the damage taken by your companion against any attacks that can actually harm it (anything that isn't an attack roll - most notably AOE damage).
Sorcerer 1: The Shield Spell. This is just one more layer to your defenses.
Sorcerer 3: Blur. Again, another layer of defenses - this time coming at the expense of concentration (so you must decide if you want more offense from Conjure Animals, or more defense from Blur).
Sorcerer 3: Quicken Spell. This gives you a more expensive, but more powerful use of your bonus action than Spiritual Weapon. It is an asset in difficult fights.
Battlemaster 3 + Ranger 5 (and a permissive DM): This will give you something to do with your leftover attack when you have your companion attack.

Prioritize any of the above abilities in whatever order you like, depending on what abilities you value more. The highest priority are the Cleric and Ranger levels. With just the Cleric and Ranger levels, you become a DPR machine that can out-tank anyone else on the field under most conditions. The Sorcerer levels just help you excel even further in both aspects.

Always thought the idea of riding the companion were to dual wield lances? :)

[Giant2005]

Always thought the idea of riding the companion were to dual wield lances? :)

If dual wielding lances is all you care about, then don't bother being a Beast Master. Just get yourself a daisy-chain of cheap, disposable mounts of varying sizes. Use whichever is the biggest mount available that is small enough for whatever task you have in mind.

[Specter]

I don't think Sorcerer is necessary for this build, since it requires Charisma and doesn't offer a whole lot. But what was said above is solid advice.

[Giant2005]

I don't think Sorcerer is necessary for this build, since it requires Charisma and doesn't offer a whole lot. But what was said above is solid advice.

It is either that or more Cleric levels, and the Cleric levels aren't really good for anything other than spell slots. If you can afford the 13 Cha, then you may as well go with the Sorcerer levels to get those same spell slots as well as sorcery points (and a couple of very useful spells that aren't reliant on having a high Charisma).

[SMac8988]

I'm not a big fan of the build above. I do see its benefits but feel it doesn't really feel like what the beast master is. It just feels, idk, not rangery.

My build I have been playing is a beast master/assassin halfling, with a wolf. Took archery fighting style, and Sharpshooter asap. Then buff up dex the rest of the way, get mounted combat if you feel it will help and feel out the feats.

I use my bow for ranged attacks, with sneak attck, then either bonus action to hide alone, or if mounted for a dash action to keep my distance.

If pushed into close combat, wolf attacks first with advantage and possible knock prone, and I attack with advantage due to Allie near by for more sneak attack damage.

I have had good damage output, still have a very ranger feel and have good skill spread and stat spread for rp sides of things.

Been one of my most favorite characters to play in a long while.

[DivisibleByZero]

Ignore Beast Master and use Circle of the Beast from DMsGuild... That's the best way to do it. Not that BM is terrible (it's actually nowhere near as bad as people think), it's just that CotB is better.