Currently Online 
God damn this is a tough one! I personally have to go with Halo. Loved the first Gears and the marketing that led up to the games release, but the game ultimately was a massive tech demo to showcase what the 360 was capable of. 2 and 3 is where Gears hit the stratosphere in the storytelling department although they did lack more in the first games horror elements.
But Halo hit like a brick right at release. That main menu music, the multiplayer LAN parties, Master Chief, the Covenant, the biomes, all on this insanely cool ring floating in space. And somehow, Bungie vastly improved some aspect of the series with each new game. Halo 2 was my very first midnight launch and I didn't experience that kind of high again till Halo 3.
You called down the thunder, now reap the whirlwind
