FFVI was mainly the character themes I loved. Each character, no matter how petty they were in the game, had a very distinctive theme unlike any other FF game I've played.
However, the soundtrack for FFX tops it in my opinion. Otherworld, Hymn of the Fayth, Suteki Da Ne (spelling?), are songs you can play at almost anytime and it takes you right back to the game and the feeling you get when you first saw those respective scenes where they first played.
It would definately have to be FFX...........the music there would stick in your head for a long time......i think FFVI would be second......its character themes are good especially Terra's theme........I used to just stand out there on the world map just listining to it lol.......but the Ending theme in FFX will leave you crying.