Any infinite-respawning enemy from the cod series, it's like your weapons are completley pointless, you shoot someone and in under a second someone else takes their place, so the only way to play the game is to just sprint between checkpoints (at which point your teammates instantly take the mall out), ignoring your weapons entirley.
Also I don't know if it counts as an enemy or not but Lou at the end of guitar hero 3 pissed me off no end! Just had to keep playing over and over again until I got lucky with what powerups both he and I got.