I recommend:
- Fable II and III
- Gears of War 1 and 2
- LIMBO
- Super Meat Boy
- Perfect Dark
- Banjo Kazooie and Banjo Tooie
- Shadow Complex
- The Maw
- 'Splosion Man
I would start there. Remember you can try the trials of all games on XBLA, and there's a lot of stuff there, so you might wanna just look around it for awhile and see what appeals to you. Fable II also has a trial that let's you play the first hour or so absolutely free, so I'd check that out. I've played a little Viva Pinata, which I think is pretty good, so you might wanna check that out as well.
Congrats on your purchase!