I’m going to have to say FF6, because I played that game to death. Shitload of characters to use, lots of espers to equip, and dual wielding Illumina swords ftw.
A first runner up would be FF7, only because of the wow-factor of 3d graphics and cut scenes. I played the hell out of it during college. It kept me sane.
Not an FF game, but it’s an honorable mention. Chrono Trigger. It’s second only to FF6 in terms of how much I played it (hell, I probably enjoyed it more). Found every secret, every item, every tech ability, completed every ending, etc. Loved that game to death. Must have played my “+” game through at least a few dozen times or so (the first go around). Everything about that game is awesome.