Probably the biggest chapter would be TF2, but there still is L4D/UT3/CoD. Just mainly depends on what kinda gameplay you're looking for. Classic death match, fresh experience systems and leveling up, large team based matches, or smaller squad-based co-op.
I want to get red faction sometime.. but for now I have KZ2 and Warhawk, both of whihc are great and have decent clan setup (i think...?)
I'd say LBP but my dsl has issues with running it, i can barely do 2player online.