Personally, my three favorites are VII, X, and XII. I love all 3, however for number 1, I would have to choose XII. I love the amount of freedom you have to explore without having to worry about random battles. You can actually see your enemies, and you fight them without transferring to a battle screen. Also, all the sidequests, hunts, and hidden bosses as well as all of the espers just added to the experience.