Probably Aeris's theme (Aerith's Theme in FF7 OST) or Tina/Terra's Theme (especially the version which is called Awakening in FF6 OST). Other great character theme songs are Lenna's Theme, Celes, Tifa's Theme, Cid's Theme (FF7) and Garnet's Theme (a version of Melodies of Life). Auron's Theme is also pretty cool.