Uncharted 2 gets my vote. The single player is going to be awesome, but don't underestimate the MP. A lot of people assume the MP as an afterthought, but we're talking about naughty dog here, I played the beta and it was addicting as crack. MW2 is great, but where else can you climb up a building and throw someone off and instantly kill them in a MP? Good shit.
Of course If I could choose one for each platform, MW2 gets my 360 vote, and maybe NSMB for the WIi.