I don't own any of those games on the PC, but I own Borderlands 2 and Mafia 2 for the 360, and Penny Arcade 1/2 for the PS3, so I think I can still tag along in those games even if I can't play along in game.
It's tough for me to pick just two PC games, but I guess I'd choose...
Stubbs the Zombie in Rebel Without A Pulse
Legend of Grimrock
I know not a lot of people will have Stubbs, but it seems like such a left-field type of game that I think it'd make an excellent talking point.