Depends on the context. If it's a super-epic confrontation with cosmic magic being flung around, there can be nothing other than an instrumental version of Our Solemn Hour: http://www.youtube.com/watch?v=KjGz9aH_05U
If I'm some sort of spectre from the hero's past; a mentor turned evil, an old nemesis, a one-time best friend, it has to be "I Own You" by Shinedown. http://www.youtube.com/watch?v=WTsl3xTwC9o
But let's face it, final music is never going to get any better than the two themes from The End Of The World, in the original Kingdom Hearts; "End Of The World" and "Fragments of Sorrow"
http://www.youtube.com/watch?v=KhdlCRXX6dw&feature=related
http://www.youtube.com/watch?v=cdK5XAVGzww