I haven't played Super Mario Galaxy, but even if I had, I might not vote for it. Yeah, I've heard the music on YouTube. Somehow seems different from usual Mario fare (besides that it's orchestrated), like maybe it doesn't belong. Doesn't change that it's good, but... something's missing. Can't vote for Smash Bros. Brawl either since that's not out yet, although the Mario tunes there are the best I've ever heard.
So I'll say Super Mario RPG.