I think the best games on the console have already been mentioned. Also, Flower and Flow are 2 games that are slightly more obscure for download and actually make use of the Sixaxis. Might be worth checking out for something a bit more unique.
I'm guessing you'll have most multilats on 360?
Uncharted 1 & 2 (based off the beta)
GT5: Prologue (or you could just wait for GT5 I suppose, but download the GT:HD concept in the meantime)
MGS4 (if you're an MGS fan, otherwise might be a bit too much story-wise)
Resistance 1 & 2 (The co-op in 2 is particularly good)
Heavenly Sword (short but I think its cheap now, not worth a full price-tag)
LBP might be worth getting, not my cup of tea but everyone else seems to enjoy, lol.
Demon Souls is worth checking out by all accounts as well.