5. Smoker - Left 4 Dead
This is simply one of my favorite co-op games of all time, mainly due to it being made by one of my favorite developers and having one of my favorite monsters... zombies! This hit for Valve had you working together with three other people to escape the zombie apocalypse, which is nice. If all the zombies weren't enough to contend with in this game, you also had the added problem of the special infected to deal with. These special infected too the forms of the Hunter, Boomer, Smoker and the Tank. I've picked the Smoker out of the four of them due to it being highly annoying yet extremely scary. You knew it was near by its wheezing and labored breathing, but the majority of the time you couldn't see it. Just when you thought you were safe with the exit door in sight - especially on the hardest difficulty - one of these Smokers would just appear out of nowhere and ambush you, often making you yell out in surprise and annoyance. They were also really, really ugly, covered in tumors and having a tongue that could put Gene Simmons to shame.