The soundtrack on Alan Wake was good, but I felt that was more they chose good existing songs to use, specifically at the end of the episodes. There was a heavy usage of Poets of the Fall in there. I don't remember much of the original soundtrack created for the game though and ultimately, I feel that's what has to win a true soundtrack award.
Red Dead set it's atmosphere very well, it had one particularly rememberable tune, but I can't remember off the top of my head how it goes.
Mass Effect 2's soundtrack was pretty good.
Especially the 'Suicide Mission' track that is used at the start of the game and the end. Very cinematic feel to it and wouldn't be out off place in a Sci-Fi movie, I'd also argue that Mass Effect 2's game opening is probably one of the best I've ever seen.
Ultimately though, I'd probably give the award to Halo Reach.
It's the most evolved and I'd say the best of the Halo soundtracks. Couple that with the fact that they've got a huge budget and spanked it on getting a proper orchestral back up.
I think it's also quite a clever soundtrack, because it's got quite a militaristic feel to it, which begins by being quite bold (as in to say, "This is Reach, home of human's Military Might") but the Trombones (I think its that instrument, could be wrong) become a lot more dreary sounding as the game goes on, to reflect the fact that the humans are loosing.
It's also a good enough soundtrack to just listen to in it's own right and I've had it on my iPhone. So that for me instantly says it's the clear winner.