THE multiplayer game for me. I've had some of the most fun goofing around with my friends while on this game. A full Versus game with everyone in VC screaming at each other is just pure chaotic fun. It's also dirt cheap while on sale, which helps. A lot of your friends will probably just have this in their library by default because of that. Which means more people for that full Versus game. And more people to scream at for missing that charge. I adore Left 4 Dead.
Also, mods. The workshop scene in this game is nuts.
Glubtastic has driven me insane