I'm gonna go with the Halo 3. We play the shit out of that game in my house. All I gotta do is watch one of my kids playing online for a couple of minutes then I'm wanting to play. So much variety playing online with all of the different playlists, double exp. weekend games and Bungie spitting out new maps. These are just a few things that give Halo 3 so much longevity in my house.
After the Halo stroke fest, I'll say that Gears 1 (I'm sorry but outside of Horde mode, Cole's rap, and a few of Marcus' one liners, Gears 2 disappointed me) and Left 4 Dead have been my favorites although I have yet to play many of the games that a lot of people like.

Dead or Alive 4...

I haven't played or completed many 360 games so far but I can tell Lost Odyssey is my favorite so far. Then it's Fable 2. That might change once I get my hands on SO4 and Tales of Vesperia. :3

still gears of war 1. it trumps everything else on the console imo except maybe gears of war 2 and resident evil 5.

Viva pinata, followed by Oblivion and Mass Effect

Well my top 5. They are all really close though.

1. Mass Effect
2. Halo Wars
3. Halo 3
4. Gears of War 2
5. Fable 2

1.)Tales of Vesperia
2.)Ninja Gaiden 2
3.)Lost Odyssey
4.)Devil May Cry 4
5.)Mass Effect
6.)Eternal Sonata
7.)Call of Duty 4
8.)Burnout 360
9.)Street Fighter 2 Turbo: HD remix
10.)Blue Dragon

Right now Banjo-Kazooie: Nuts & Bolts. Will most likely change soon.

Very touch choice between Braid and Left4Dead

Bioshock, Braid, Gears 2, and Bully.