The only 360 game that kept me coming back for quality MP for at least 8+ months was Mass Effect 3. I'd say that's by far the best MP I've played this gen.
Another good one (PS3 though) is Untold Legends: Dark Kingdom. Played that co-op/mp for at least a year.
I had my share of great FPS MP on Xbox with Halo 2 and early 360 with the first couple CoD games. After that they just plateaued and I've steadily lost interest in them, to the point where I'm not sure what it would take to pique my interest in the genre again.
Still play Animal Crossing Wild World with my buddies. Love that game. Some others: Super Smash Brawl (if only they has a passable online...) Mario Kart 7 WoW Halo 4 Forza Horizon Halo Wars Battlefront 2 (wish the online was still up) Starcraft 2 Diablo 2 and 3. Warcraft 3: TFT
That's about it at the moment. Assassin's Creed 3 and Fable 3 get a shoutout, though
Here comes Old SunDevil77, he's a scaly man-fish. You don't know me. You don't know what I got. I got something to show you.