If this has been a topic before I apologise in advance...
So yeah, self-explanatory really. Was considering doing a poll but then I remember there's about 80 or 90 different pieces in this game, so...
Id (Sorrow) is probably my favourite. It's so soothing and it fits the tone pretty well considering when it's played. Conquest (Ablaze) is a pretty close second, although all the ablaze songs from what I remember sound good