Any cheap enemy from any game you hate

[Espio]

Personally mine would have to be those damned Goil Menhirs from .hack infection (If you've played that series, if not >signature), they can revive killed monsters within range unlimited times to my knowledge and also revive their own kind, making it a pain in the ass to fight only them.

[Mooseknuckle]

Medusa Heads from the Castlevania series.

[/thread]

[Medicris]

THIS GUY

RIGHT HERE

screw his level too

[Ivan]

Hard Doom lost souls, gg.

[someplayer]

Creepers from minecraft.

[Infirnex]

Mask of Evil in The Binding of Isaac: Wrath of the Lamb.

Every. Damn. Time.

[Combinebobnt]

Fucking bats from any game ever

[CloudFlash]

http://silenthill.wikia.com/wiki/Needler <--- Theese guys. OMFG. They are fast, they are quick in close combat, they can block shots from pistol using their blades, they often crawl on ceilings and catch you off guard... ;=;

[Catastrophe]

.

[Konda]

Super shotgun guy, archvile, enderman from minecraft.

[Zeberpal]

Snakes from Resident Evil 4/5. Always gives me heart attack.

[Bloax]

The AI from Worms games.

Oh yeah, that grenade that you threw from god knows where, which is currently residing on my head? Yeah, that one seemed very legitimate.

[♠Enigma♠]

THAT BIRD in Ninja Gaiden 1,2 and (I think) 3. At a certain spot, those things constantly re-spawn.

A close second is that Trap Door in Final Fantasy IV

[Thetis]

The Death from Gauntlet drives me nuts when I see them on the map.

[The Blue Panther]

The rats from SiN, they're so annoying you can't be near them without your character going all "Yeargh!", and basically any little insect that wants to kill you in any game..

...And Chaingunners.

[Cerebus]

Hunters from the Halo series. Absolute pain in the butt to kill. >:)

[Laggy Blazko]

Well, I try to be cheaper than my enemies. =P

But... MOHAA snipers and their almost perfect accuracy, I guess...

[Qent]

Freaking Dragons.

[z-blaster]

This guy from Prince of Persia Sands of Time. You Can't jump over him and he sweeps his sword at your feet and hurts me even when I block

[Devon]

Big anneid arachnids from System Shock 2

It seriously takes 15 normal bullets to take one down, if you have anti-personnel bullets then that's considerably less, but they're hard to come across. They also make you use all your anti-toxin hypos, which is a bitch as they're pretty rare, too.