I have to say Halo 3 Live multiplayer is the best. It's so damn fun, it's basically non-stop action, plain and simple. I do amazing sometimes when I'm high too, that's how I got my overkill on FFA achievement. Plus, I always play music through my 360 while I play.

Counter-Strike: Source is really fun, but I get bored waiting between rounds.

Warcraft III custom games are fun to do while watching tv, too boring otherwise.

Burnout: Revenge is really fun and intense, but it doesn't compare to halo.

Skate I bet would be really fun.

Guitar Hero