I'd have to say IX is my all time favourite. Wonderful art design and a brilliant main character who wasn't a whiny, moody teenager. Also, Vivi is and always will be, my most cherished FF character.
After that, I'd say XII was my next favourite simply because the gameplay was awesome. There was just so much to do and find outside of the main story in XII - all those bounties, ultimate weapons and additional summons to find among other things, which was a real treat after the slightly more anaemic affairs that were X and X-2.
I also really liked being able to actually choose my characters weapon sets, which allowed for a massive amount of flexibility in the battle system.
Unfortunately, I really didn't enjoy the new direction that SE took with FF13. So that's where my relationship with the series ended.
After that, I'd say XII was my next favourite simply because the gameplay was awesome. There was just so much to do and find outside of the main story in XII - all those bounties, ultimate weapons and additional summons to find among other things, which was a real treat after the slightly more anaemic affairs that were X and X-2.
I also really liked being able to actually choose my characters weapon sets, which allowed for a massive amount of flexibility in the battle system.
Unfortunately, I really didn't enjoy the new direction that SE took with FF13. So that's where my relationship with the series ended.