Bill Zebub
“It’s probably Matt Mercer’s fault.”
Exactly
Favored Enemy
Beginning at 1st level, you have significant experience studying, tracking, hunting, and even talking to a certain type of enemy.
You gain one of the following features of your choice.
Banisher
You are a bane to those not of this world. When you score a critical it against an aberration, celestial, elemental, fey, fiend, or undead, you deal an additional 1d12 damage.
Colossus Slayer
Your tenacity can wear down the most potent foes. When you hit a creature with a weapon attack, the creature takes an extra 1d8 damage if it’s below its hit point maximum. You can deal this extra damage only once per turn.
Giant Killer
When a Large or larger creature within 5 feet of you hits or misses you with an attack, you can use your reaction to attack that creature immediately after its attack, provided that you can see the creature.
Horde Breaker
Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.
Lone Stalker
Seclusion sharpens you ire. When you hit a creature with a weapon or Unarmed strike, you deal an additional 1d10 damage if there are no allied or hostile creatures within 20 feet of you. You can deal this extra damage only once per turn.
Man Catcher
Your weapon attacks and unarmed strikes against giants and humaniods score critical hits on a roll of 18, 19, or 20.
When you gain this feature, you also learn one language of your choice.
You choose one additional favored enemy feature and an associated language, at 6th and 14th level. As you gain levels, your choices should reflect the types of monsters you have encountered on your adventures.
So...without going into whether or not those abilities are well-designed mechanically, in my opinion they demonstrate the problem with the "favored enemy" concept. While each (or most) of them carry some flavor suggesting "favored enemy" none of them really implement it. And I don't mean this as a criticism against your ideas, just of the futility of trying to force fit broadly applicable mechanics into the "favored enemy" concept.
Banisher: six different categories of opponent? So, brownies, liches, devas, beholders, and gelatinous cubes are all favored enemies?
Colossus Slayer: sure, colossi tend to have more HP, and thus are more likely to be below max HP during a fight, but this ability also affects tiny creatures missing one HP. So favored enemy is "injured creatures"?
Giant Killer: if "Giant" refers to size, and not creature type, this one somewhat narrows down the field, but is still quite broad.
Horde Breaker: so, "favored enemy" is any enemy that isn't completely by itself? A drow riding a giant spider, for example, would still qualify.
Lone Stalker: this says nothing about the enemy, just the Ranger's modus operandi
Man Stalker: this is probably the optimal pick, in the absence of other information about the campaign, just because it covers so many creatures you are likely to fight: bandits, evil wizards, orcs, goblins, drow, other PCs (!?!?!), and of course every type of giant.
I have no problem with giving players a choice of fun abilities (although in general my preference is for abilities that result in different decision-making, not just ones that make you better at what you were going to do anyway. For example, I like...and have thought of the idea myself...of getting an opportunity attack against Large+ creatures when they miss you, because it encourages archers to move in close). Again, my issue is just with continuing to try to force fit abilities under the name Favored Enemy. We (and WotC) should just give up on that.
Unless, that is, the favored enemy of all Rangers is simply "monsters" and is implicit in the class design. And then I would design the abilities so that they help the entire party, not just increase the damage of the Ranger him/herself.