Impossible to decide on a single piece, too many amazing ones.
My top five in terms of:
- epicness:
1. Fate of the Unknown
2. L'Impeto Oscuro
3. Darkness of the Unknown
4. Black Powder
5. Rage Awakened
- character themes:
1. Ventus
2. Roxas / The Other Promise
3. Aqua
4. Riku
5. Vanitas (Enter the Darkness)
- background themes:
1. Link to All
2. Tears of the Light
3. Fragments of Sorrow
4. Friends in my Heart
5. Always on my Mind
- world field themes:
1. Radiant Garden (BBS)
2. Land of Departure
3. Land of Dragons
4. La Cite des Cloches
5. Traverse Town (DDD)
I could still make up more categories, lol, Yoko's songs are nearly all outstanding.