As said before top 5 is almost impossible, but I'll list what stay in my head after I play:
1. Y's Book I & II > Some of the most awesome music ever.
2. Devil's Crush > Hot damn what a track, if the game itself was crap, this track would make me put it on just to hear it play.
3. Gate of Thunder > Music feels so perfect for this game, it is just fantastic.
4. Lords of Thunder > Like heavy metal magazine come to life this game and its music are stunning.
5. R-Type > Fantastic version of the arcade music, really love it.