Lol, if you're talking about the one I think you're talking about, that
(SPOILER)Clash on the Big Bridge (I think from FFVI). I know for sure it plays while you're
(SPOILER)fighting Gilgamesh. Yes, it's awesome, but it was originally done by Uematsu. And that's my one big problem with the FFXII OST. Pretty much all the music on there I like is a re-arranged version of a Uematsu piece. The new composer is obviously good, but it's more music to suit a certain situation. Uematsu's music was brilliant to listen to just by it's self. It had brilliant and very memorable melodies. I often found that his music for when you are simply running around the environments was not so good, but the character's themes and the music for the big events in the game story-wise were always brilliant. And that's what I miss from the new soundtrack. Shame.
The other piece I really like is the FF prologue (number 2 on disc 1), and guess who wrote that...
Be more careful with spoilers - Psychotic