Hell; one song? =\ Well, if it comes to that, I doubt I could properly sleep at night were I to offer up anything but the Advent Children rendition of "One Winged Angel". Just entirely too much raw compositional and instrumental power to be denied. In terms of how thoroughly it fills out the soundscape, one would be hard pressed to find a rival in any musical genre, methinks . . . And it is certainly a feat worth noting that Uematsu managed to add an original verse to the existing piece and invent entirely new lyrics to be set against the old music . . .
Peripheral choices would probably include the surreal and utterly despondent "Wandering" from the Final Fantasy X OST, perhaps "Servants of the Mountain" from the same soundtrack, if the mood is conducive thereto, and perhaps "Judgment Day" from the FFVII soundtrack. Three other possibilities just sprang to mind, though, so this post will be summarily be put to death before it has a chance to turn on the hangman and rally the townspeople. <_< >_>