Mario Kart Wii.

Halo 3

Left 4 Dead is by far my favorite multiplayer experience. I am not normally all that interested in online play, but the coop campaign with some good friends makes for endless fun. That is without even counting Versus, which is not one of my favorite features but for competetive players it is there.

Left 4 Dead (PC version), LittleBigPlanet, Mario Kart, Warhawk, PJ Monsters

Gears 2 might be a contender as well, but I haven't finished it yet.

2 words: resistance 2
This game, while maybe not being the most graphically polished on the market, is the most fun i've had in an online FPS since the original halo. I think since your getting a ps3, then this is the game for you! The co op online is very fun and will keep you busy if you get tired of the competitive, which i doubt. And since you like to skip straight into the mulitiplayer, this game is perfect for you since the single player isn't really that good lol.

Why only pc version?

From what I've played, the PC version is faster paced, especially in versus. The x360 versus games I've played are slower, clunky, and just feels different. This is primarily due to the controls. Also, PC gets bug fixes and gameplay balance changes faster. One major balance/bug fix update took 5 weeks to show up on x360 from when it went live on the PC version. If i'm not mistaken, xbl would employ p2p while the pc version employs primarily dedicated servers, in addition to p2p, which nobody uses.

COD 4 (seriously, would it be fair to reward bad players with air strikes and Helicopters?), but the best multiplayer game I've played is Warhawk. Better than Halo 3 (just my opinion).

Team Fortress 2 followed by Warhawk. Left 4 Dead might make its way up there, but I haven't gotten around to buying it.