FFVII, with the only real contender being VIII.
VI was great, really great, but nothing was really that memorable for me, need to play it again.
IX was just dull, it took no thinking or skill at all you just went along with it having no real control.
X should have been great, but instead of taking advantage of the technology they just made everything that had been great, got rid of it and replaced it with good graphics. Mini-games were torture, and having an underwater game and making it 2D just highlighted how little care was taken over gameplay. I don't care how windy a path is, if it's a foot wide it's gonna be dull whatever you do.