9. Witches - Left 4 Dead 1 & 2
If there's one extremely vital rule you must follow in the Left4Dead series it's this:
Do. Not. Mess. With. Witches.
Witches are the bane of any L4D fan, be they casual or pro simply because nine times out of ten, one simple mistake will lead to a team falling apart at the seams. Signified by their haunting wailing Witches can come in two forms, a static seated version and the much more dreaded walking variant which means that getting past her is even more arduous thanks to the unpredictable movement.
However what makes The Witch such a subject of contention isn't actually anything to do with its design or its behaviour because it really is a well designed enemy, it's more down to the fact that Witches will make you hate your teammates. Whether though mistimed moves, a rogue bullet or triggering an alarm you will curse loudly and in a shrill terrified voice when your mates give The Witch a reason to give chase.
She hits like a truck, instantly incapacitating you, and will cut through you like you're make of wet bread. You'll be left on your back cussing out your team until you bite the big one.