I'm going to have to agree with everyone and say L4D, it really is the best Co-op game I've played because its not just an option added afterwards, the game was designed from the ground up around the concept and in doing so has somewhat become the benchmark of how it should be done. You really NEED to work together to succeed, if you wander of you'll end up Hunter or Smoker food and be completely powerless to stop it!
It seems almost gimmicky to write about it, needing to help your team mates, but it works so well with the game you don't even notice it when playing, and it FORCES teamwork.
I mean if you lose 1 player at the start particularly in Versus where theres no respawn and smarter zombies, 9/10 its over and you're merely trying to get as far as you can to get your score higher before you inevitably die.
Half Life
17/10/2009 / Melbourne, Victoria | Wake up, Mr. Freeman. Wake up, and smell the ashes.