Viva Piñata

Banjo, but not because of Microsoft.

In fact, Microsoft didn't have anything to do with the good parts of Banjo. Otherwise, going with something that was all Microsoft, Age of Empires easily. Or Madness, that was fun. Midtown Madness but especially Motocross Madness.

Where is Minecraft?

Age of Empires.

Forza (except H2), Halo, Gears or Fable (Assassin's Rush is still my fav power in any game).

Fable because I liked Fable I, really loved Fable II and Fable 3 was kinda ok, not interested in the Kinect game or the upcoming Legends but would be interested in a Fable IV if they ever bother to make one.

I like user generated content and new IPs so I pick Project Spark.

I can't say Banjo or Conker because the good games in those franchises came out before MS bought Rare. Taking that into consideration, my answer would have to be Halo.

Halo with Gears of War a close second.

Like a few here are pointing out: add Minecraft into the poll