Halo Reach defs, you don't think twice on Halo.

But it all depends on what you like. If you like shooters, two other games would have to be Medal of Honor and Bad Company 2(or sub one for a Call of Duty Black Ops pre-order imo).
As for RPGs, I'd say Fallout 3 GOTY or Fallout New Vegas, and also Mass Effect 2.
Lastly if you want some action/adventure type shit, I'd suggest Dante's Inferno and Bayonetta which although aren't exclusive imo run better on Xbox(especially Bayonetta)