Best Xbox 360 game to date?

What game do you feel is the best game out of every game to ever come out for Xbox 360? It doesn't have to be exclusive, just any game out on Xbox 360. Just your opinion, not arguments.... I feel it is Gears Of War hands-down.Best Xbox 360 game to date?
GTA IV or OblivionBest Xbox 360 game to date?
Oblivion or Mass Effect
COD4
i would say cod 4 or halo 3.
bioshock. end of story.
Mass Effect or The Orange Box for me. I think Mass Effect is my favorite game. However, if I had to choose one of the two, I'd pick The Orange Box, since I play it more than anything else.
Oblivion or Bioshock.

I really loved both of those games.
Mass Effect.
TOP 5 BEST GAMES TO DATE:1.GTAIV 2.COD 43.Halo 34.Bioshock5.Obilvion
Grand Theft Auto 4

Best exclusive- Halo 3
Halo 3 and Oblivion.
Forza Motorsport 2 TDU or GTA 4.
Mass Effect or Bioshock
I'd have to say BioShock, i thought it was one of the best games i've ever played.
It's hard to pick one over the other because they are so different but I'm going to go with Bioshock over Oblivion.