
Abilities like sneak attack, some battle master maneuvers, and colossus slayer, and spells like hunter's mark (as you already pointed out) also have great impact on damage from and for AoO. Great Weapon Master and Polearm Master specifically and positively effect the frequency of AoO, and thereby additional damage, while any ranged build (Sharpshooter and Crossbow Expert) completely remove the occurrence of AoO (and your hit point pool to the party resources).

I know a lot of folks don't, see WolfOfTheBees above for "guaranteed damage", but I think that is a flaw in all of the white room, perfect scenario, theory crafting DPR that we all do. Personally I am one to take things like attacks of opportunity into consideration when looking at average expected damage. In the end i think i will take the Giant Killer anyway, just because it's fitting the best for my character. What is your opinion about this and may have you a homebrew solution or something. This could be huge, because Hunters Mark is triggering, which isn't the case for Horde Breaker, but it is rather situational. And then i can use my reaction to attack it back afterwards. Now, for my giant killer to trigger, i need my reaction, while standing in 5 feet to an enemy that must be large oder larger than that., which has to attacking me.

And for Horde Breaker i "only" need one enemy, that is adjustant to the one i'm attacking. As a Colossus Slayer, the enemy has just to be injured, so i do additional 1d8 damage. For the Monster Slayer Subclass, i get 1d6 bonus damage, but i have to use one Bonus Action, that i want to use for Hunters Mark. So in comparison for a Ranger with a Greatsword, there are some options i could take. I want to play an Giant Killer Ranger, created all my backstory for a Lvl 1 Ranger. Monstrous Compendium Volume One: Spelljammer Creatures
