Most of the soundtrack, but especially "Small Two of Pieces" and "Creid."
The Orchestral version of Small Two of Pieces is even more heartwrenching, especially when edited into the ending FMV.
For those who like darker themes, "Omen" in particular is something of a masterpiece.
VGM remixer Star Salzman has melded "Omen" and "Dazil: City of Burning Sands" into a dark, powerful masterwork known as "Pillar of Salt." Listen to it here: http://ocremix.org/remix/OCR01225/
And those who enjoy epic boss music should definitely check out "Awakening".
"Light from the Netherworld," the background music to the opening movie. Incredibly eerie and chill-inducing. Even if you've never even seen the opening movie, you can tell just by listening to it that something inexplicable and unearthly is going on.
And now the Orchestral version is out, edited into the Opening FMV.
Id's theme, "The One Who Is Torn Apart." It can only be described as haunting.