It is a crazy four way tie between Xenogears, the Shadow Hearts series (Yoshiitaka Hirota is my drug of choice), Tomba!, and Spyro.

Out of the four, I think Shadow Hearts wins by just a smidgeon. The Astaroth song is probably the best boss song I've heard, EVER.

Edit: They censored Hirota's name! I had to add an extra 'i'.