Boss battle music can make or break a game from an aesthetic point of view. It has to capture the intensity and drama of the fight as best as it can. I'd post this in the Mario games board and ask the same question but currently my favorite piece of boss battle music is from the Molgera battle in TLoZ: The Wind Waker. The HD remake of this song is amazing too, but I feel the original captures an "underdog hero" feel better. Feel free to share your favorite music from any boss battles you've come across in gaming.