J-E-N-O-V-A gets my vote, after a loooooot of deliberation. I've always liked the song but it's never been my absolute fave boss fight music. That intro though is amazing.
Valley of the Fallen Star is so good, I've had it stuck in my head many times and can listen to it often on repeat. However I voted for The Great Warrior in poll #8, which I think is the 'better' song of the two by a somewhat narrow margin. So, eh, J-E-N-O-V-A gets my vote. :3