Even though I'm a Nintendo fan, I think almost everyone will agree that their online sucks, so none of their games get my vote. Anyways, for me Halo is the clear winner. Sure, there were some people I played with who were jerks, but the most part it was a lot of fun (plus you can always just turn off your headset if it gets really annoying).
BTW, multiplayer doesn't need to be online. Mario Party is a classic example. No online, but still good fun.