I vouch for VI.

Though simple in design, it was an enormous game at the time, and all the environments served to emphasize that. I found the battle effects to be amazing then and now, and the story was superb.

The best part? This game had more characters than VI, VII, VIII, IX, AND X, and still managed to give each of them a chance to shine in the spotlight. What's more, the main villain amused me more than any of the ones in the later games.

This is the one that started it for me, and still my favorite.