I really do like all of the music, but from the original games, it would have to be the forest dungeon music, and then the Drain Heart track, the one that plays when Kite destroy Corbenik at the end. >:
Then from the G.U. soundtrack....Well, i /technically/ dont know the name of it, but it plays at various parts in the game....then there is this one too.
http://www.youtube.com/watch?v=1qmxeVDJZYoThen my next favourite would have to be "May I help you?" xD