http://www.youtube.com/watch?v=TS7KeCS0ub4 A group of individuals without hearts, Organisation XIII, consisting of (surprisingly) thirteen members, is one of the series main adversaries, though some of their members can be viewed with sympathy as tragic figures (Axel chief amongst them). This musical accompaniment, used in several other excellent themes (including Xigbar in Dream Drop Distance and Riku Final Battle in 358/2 days), emphasises the tragic element, and though short, intrigues throughout.
2. Hollow Bastion (Kingdom Hearts)
http://www.youtube.com/watch?v=sPtg8pHJ1VY One of the final levels in the game, Hollow Bastion features such a superb backing track that I personally left the game running on a frequent basis whilst doing other things just to hear it. Retoolings of the track in subsequent games (in which it the world comes to be known as Radiant Garden) cannot top this original, which is both melancholic yet mesmerising.