Final Fantasy VI- Ok. Gameplay was boring for the first part of the game, but got more interesting when you got espers/aeons/summons/GFs/whatever the **** Square decided to call them in this game. Too story driven.
Final Fantasy X- Horrible. Probably the worst game I've ever played. Too easy and linear. Sphere Grid didn't add anything to developing characters. Bad voice acting. Music and Level Design were bland for most of the game (though, some of the areas I thought were actually very good). Too story driven.
Final Fantasy XII- Great. Gambit system sped up battles and meant you didn't have to mash X all the time. I hope the system is expanded in future games to allow for more strategy to be used. I liked the License Board because it allowed you to develop your characters however you wanted to. Music was ok. I liked how it didn't focus on story too much (though some of the cut scenes were to long, in my opinion).
Those are the only FFs I've played. I don't think I'll play the XIIIs or go back and play the old FFs.