Simple answer, it's Halo.
I used to play Halo 2 online, and it was a very fun game. It had a completely different feel to it than most online shooters, due to its concentration on terrain and battle tactics, rather than twitch shooting.
It really was, finally, an online FPS designed with a console in mind, that actually stacked up against PC shooters, in that it wasn't an apples to apples comparison. It gave online shooter fans something different, and it gave shooter fans a reason to play a game on a console, because of the design of the game, pinpoint controls took a backseat to environment, atmosphere, and general craziness during play. It was an excellent online shooter that wasn't really hampered by a console environment, due to its excellent design.
I stopped playing Halo for a few years, and the other day during the superbowl, we all plugged in ODST(or whatever, not my game) at my friends house and hit up the multiplayer. We went to the level with the big wheel in the middle that can take you up to the invisible powerup(best shooter map ever, imho). I think it was called Zanzibar in the first game, and the truth is, we played it all the way through the superbowl. It was just so great talking shit, but the game was also very addictive and easy to pick up and be good at, while not requiring as much practice as it did simple ability to think and understand what you're seeing.
We talk a lot of shit about Halo, but honestly, most of that is the anti-community, or those who never really enjoyed Halo enough to actually know if it's went downhill, or is still as fun as it once was when Halo 2 released and really XBL began.
Yeah, it's still pretty much the same game, and it doesn't look much better, and the SP is nothing to write home about, but team slayer on Zanzibar still kicks the shit out of any multiplayer experience I've ever had on console or PC, so take that for what you will.
I don't need your console war.
It feeds the rich while it buries the poor.
You're power hungry, spinnin' stories, and bein' graphics whores.
I don't need your console war.
NO NO, NO NO NO.