I'd say the future theme from Fastball 2, or the title music from Little Hero but that would be bias of me! Of course, the chiptunes to be found in the soon to be released INC will be up there... Other than that, surely it has to be the Streets of Rage soundtrack? Or I always thought the Ragdoll 2 sounds were rather fitting!
I think these are all already mentioned but: -Robot Unicorn Attack! -Superbrothers Sword and Sorcery -Battle for Wesnoth -Tilt to Live -Osmos I dont understand why people like Space Miner music so much. It was irritating and made me stop playing after 30 seconds. Havent touched it since.
I like all of the iPhone tracks, and only some of the Arcade mode tracks. The iPhone mode OST has more melody and would work better as standalone music, while the Arcade OST is meant to always have explosions and stuff as part of the sound, I think.
Honey tribe: Colony Collapse has dynamic music and all the SFX and different layers are in time. So the flowers you collect make the melody which plays over the different background tracks. And the background tracks change depending on what is going on in the game. You'll see what I mean from the video. (Yes it's my own game and yes I did the music )
I think Osmos's music is sort of overrated since it's so shallow, but it's really good for making the feel of the game.
For me it's important to look at things like music in context. Same with graphics. The music in Osmos definitely helps the mood of the game perfectly. For a game like Robot Unicorn, the music is like the main appeal.