The one asshole that blocks vital hallways/paths (the small ones that are only shoulder width, or they can't be accessed any other way) by doing one of the following
A. Reloading
B. Going AFK
C. Being completely clueless and trying to shoot enemy killstreaks down
all of this while I'm being shot in the back and I'm pleading for life.
Also, the "rescuer". I.E. the person that gets the Rescuer medal the most. Anytime I get that medal I feel really guilty, because it's "MOTHERFUCKING KILLSTEALER" in a more positive light. There have been less than 5 times I've been "rescued" and I actually needed it, and when I need it I'm really thankful, but not when I'm 2 kills away from my dogs and you hop in to "rescue me" and end up flipping the spawns so the enemy ends up spawning behind us. It's a vicious cycle.
I know I shouldn't complain about killstealing, because if you think about it, nobody really knows if one actually needs help if you see them in a firefight, so your first instinct is to shoot, but it's still annoying as hell.