Well the prelude, for one, obviously. It's a simple kind of thing that can bear repeating. Terra's theme is probably maybe my favorite. And I could listen to the FFT OST for many many hours.
It's the music that plays when Biggs, Wedge and Terra march through the snowfield to Narshe in the beginning. It's one of the only pieces of music in the game that is never repeated, outside of the Dream Oath Opera. Hearing this piece of music means that you have just started a new game of Final Fantasy VI.
It's really too bad we never get to be children again. I miss those days.