It's nearly impossible to choose just one piece... But I'd say Protecting My Devotion/Loss Of Me.

Something about the piece is just so powerful, so moving. Obviously the mood of the music changes depending on which version of it you listen to, but both of them are just beautiful. One of few video game themes that can almost reduce me to tears.

This game by far has the best OST. Not just of any FF game, but I'm tempted to say of any game ever.