For games on AG, I would have to say Eridani, not because of the music itself, but because of the context of the music. Most of the game does not actually have music (but it does have some of the best sound effects around); the only time you hear music in the game is right when and after you complete a sector, but you're so relieved to hear it that you learn to love it even though the track itself isn't anything more than a few chords on a beat.
Also, it's not an AG game, but I absolutely love the menu screen music in PacXon. I'm sure at least a few of the 67 million people who have played the game just click on it to hear the music. I can't be the only one who does that.
