The elite squad

Mostly encountered in Versus Mode, The Elite Squad will always win as thoroughly as the game will allow. If you're playing as Infected, they will dodge all your attempts to harm them and will reach the end of the level so fast you'd think there were a wet t-shirt contest and free beer being offered there. If you're playing as Survivors, you can consider it a good run if you even make it ten feet from the safe room at the beginning of the level. This group of players operates like a finely tuned machine, dispatching hordes of zombies with ease and causing their opponents to quit out of frustration. If you could alter the game so that all the zombies carried rocket launchers and all they got were their bare hands, they'd still win somehow. But you'll never think that they're cheating; you'll think they're some sort of secret military AI that found it's way into the game.