First for me is obvious, not just because it's great video game music but because the moment I hear it I immediately think about the game. It's one of the few soundtracks I keep on my iPod from a game for pete sake..
But after that, there are several that get my attention.. the Credits from Portal
Then Mario..
In fact, the entire Mario soundtrack is great.. hearing it in Symphony is also awesome
Little Big Planet
PixelJunk Monsters.. actually, this entire soundtrack is brilliant
I'd have to think on it. There are several game soundtracks I actually find pretty good to be honest. A few very good. Nine Inch Nails Doom3 soundtrack should also get a mention





