Left 4 Dead
Sweet Jesus, this may be the best video game ever. You're fighting zombies and trying to get out of the city alive. You have tons of ammo, but these fuckers are fast and come in all sorts of varieties to kill you. If you turned the Resident Evil games into an FPS and made it squad based, this is what you would get. Thrills, chills and a fuckton of awesome. Play this game. Seriously.