For me its a toss-up between either the Bastion Soundtrack (particularly The Slinger's Song, although Setting Sail, Coming Home and Build That Wall were also brilliant), Sonic Generations (City Escape or Modern Chemical Plant), or Rayman Origins (Lums of the Water or Nowhere to Run). Oh, and Want You Gone from Portal 2 and the Skyrim Theme Song.
I don't think I can really pick a single song to be the best, I just thought there were so many good game soundtracks in 2011, and I'm glad to see quite a few people agree with me on some of my choices.