4. Left 4 Dead 2
The Left 4 Dead series is the ultimate co-op zombie survival experience. If you like to go out on your own and try to be Rambo, you're as good as dead. It's all about trying to get as many of your team members from point A to point B in one piece, and that requires teamwork. Standing in your way is a horde of zombies--though they aren't the typical slow moving stumblers. The standard zombies are quick on their feet, and sprint towards you at full speed, which is absolutely horrifying.
But the standard zombies are only the tip of the iceberg. The real threats are the special, player-controlled super infected, like the original game's Hunter, Boomer, and Smoker. In the sequel, things get even more hectic with the addition of the Charger, Jockey, and Spitter. Alone each zombie type isn't much of a threat to a coordinated survivor team. But when the super zombies work together, it could mean instant death for the heavily armed humans. If you're looking for a zombie title that lets you cooperate with or decimate your friends, Left 4 Dead 2 can't be beat.