FFI: The actual Final Fantasy theme is the first that comes to mind
FFII: The overworld theme
FFIV: Gosh, I can't pick. Loads of gorgeous songs.
FFV: Gilgamesh battle theme
FFVI: Searching for friends comes to mind instantly. Loads of beautiful ones though.
FFVII: Screw Aerith and Sephiroth, Tifa and JENOVA are where it's at!
FFIX: Terra or Rose of May. Difficult.
FFX: To Zanarkand. Very difficult.
CC: Uh, the opening..? I can't really remember most of the music...