IX is definitely my favorite of the FFs. I loved most of the cast and was happy to not have to be all flashy with Limit breaks doing 9,999 x1000. ATE's were awesome too.
I enjoyed X's level up system. It was nice to be able to max out stats (though it resulted in everyone being carbon copies of each other) and customizing weapons - somewhat. Having "Ultimate Weapons" that weren't automatically ultimate was pleasant for me.
Aaaannnnddd…finally. Despite all the girl power and crap storyline, etc. I liked fighting in X-2. Job classes (Yes!) in the form of dresses (...) Fighting in that was much more fast paced, especially if you weren't the type to change/keep the setting as having the enemies wait while you are in the menu.
I haven't been able to play XII because I've managed to misplace all my PS controllers.