Generally robot enemies in any shooter. Be they robot turrets, flying robots, or robot aliens (looking at you, Crysis), I just really, really dislike them. For whatever reason, no matter what game they are in, they seem to not fit quite right with the existing mechanics. They screw with the way you've learned to play the game, but not in an interesting way, always in a dull, tedious way.
On top of that, robots just aren't fun to shoot. They rarely flinch, and sparks don't carry nearly the same weight as blood spray.
On top of that, robots just aren't fun to shoot. They rarely flinch, and sparks don't carry nearly the same weight as blood spray.