Is this online multiplayer? I tend to avoid online multiplayer, the only one I played seriously being the first Neverwinter Nights. If its couch/local multiplayer then my favorite would be Castle Crashers. I guess playing Starcraft over LAN is also kind of similar in that sense so I guess that would also be a favorite of mine. I generally prefer single player games though and I generally dislike the people I meet online, they being people and me being an unsociable prick.
Also, I tend to find that assholes congregate when things are competitive in nature. Seeing as video games are ultra-competitive: Assholes.