Left 4 Dead (2008)- PC
Being able to play as the infected against another human team is also a fantastic feature. Once you get really good at sneaking around the infected and finding good hiding spots to camp out while you snack on some chips, I recommend turning down the game volume and listening to classical music- it makes for a very cinematic experience.
You can check out my review of L4D here if you want a more in depth explanation of why the game is so much fun to play.