GTA IV, no competition. I mean, my friend and I walked 20 kilometres in heat wave weather, just to rent it for a single day, and it do it all over again to play it again.
Bioshock was amazing until I realized I had to kill every Big Daddy. I don't mind killing one or two, but I don't like using all my ammo to kill them every time. Because of this, I have to say Halo 3 is my favorite XBox 360 game.
Bioshock rocked on the story mode, I loved the incredible graphics and the spot on 1960's (?) american style of architecture and design. Also the gameplay was great