Dead Rising 2 - Best with a friend. Nothing like running around in a borat-esque banana hammock, getting on the digital piss with a similarly-dressed bro and whaling on zombie filth. (See attatchment.)
Left 4 Dead 2 - Again, best with friends. The online community seems a little dead.
Mass Effect 2 - Make sure you bring that wallet so you can nab the Shadow Broker DLC, too. Apparently, the Shadow Broker DLC is basically better than the main plot. (But I wouldn't know, too tight to throw money at DLC)
Team Fortress 2 - Multiplayer definitely required, but it's possible to play offline.

I like sequels, don't judge me.