Left4Dead 1&2 is almost always fun paying coop campaign levels. Did just about everything you can in Borderlands and pretty much never want to touch that game again. I still have some fun throwing in Rainbow Six Vegas 1&2 for Terrorist hunts. Also zombies
in Black Ops, but it can get pretty boring fairly quick since there is no end goal.
For player versus player I think Bad Company 2 is the funnest.