'Charger' type enemies that have only their back or a small weak area that they can take damage from. They also got to have an unpredictable attack pattern, and move very slowly until you get near them, which then they charge. (Most recent of me seeing this is the Beserker demons in Shadow Warrior and later in the game they throw like 2-3-4 of em at you at a time with other guys in the mix.)
Enemies that can become invincible at will, and after becoming invincible, spawn a hundred little mini versions of themselves at a time for you to wade through, and by the time you reach them they've moved, burrowed under the ground, smacked you up a bit, became invincible, and spawned thirty more little buggers while another one gets aggroed to you. Case in point, Torchlight 2's Dune Mothers. Holy crap those were annoying, and they're a recentish example.
Enemies that teleport as soon as you shoot or hit at them and at will, deal immense damage, are complete bullet sponges, and teleport in front of you when you're just tryin to get some breathing room.
Flying mooks that bip and bob about and come in about every other wave of bad guys, shooting at you, and also have large health pools.
Enemies that are completely resistant to all elemental/magic damage, the bane of wizard dudes/dudettes in rpgs. Huge elemental resistances in general.
Enemies that chase you for miles once they've seen you and are hard to hit, deal a lot of damage, and also explode through wooden doors like they're made of tissue paper mache. Undead Dogs from 7 Days to Die. I know, that's an alpha game and they're being balanced and such, but daaayum are they annoying when you run into them. At least it's been made in that you don't see five every twenty feet. They were like land cliff racers. Speaking of which.
Enemies that prevent you from waiting/resting because they see you through a mountain that they're flying against. Also ones that make loud noises and ones that can kill or severely wound a low-level character and are EVERYWHERE IN THE GAME'S OVERWORLD. And only multiply their nuisance and damage when they're diseased. Once you're a flying sorcerer that don't need no maaaaan, swingin gaint electrified daedric daikatanas around you kind of
want to go out and continue Jiub's crusade against the flying menace. Till then watch the skies Outlander.
Anything that stunlocks you or can knock you over repeatedly.
Anything that has no obvious signs that they can heal themselves at will but go 'aha! I shall use this healyheal!' when you deal a lot of damage to them.
Anything that heals other enemies and becomes invincible temporarily and then moves quickly when you hit it.
er... what else...
I'm out for now have this