For me, I would have to say Final Fantasy VIII was the most enjoyable.
I understand a lot of people didn't like it, but it is one of my favorite games to date (I still every now and then do a playthrough).
I really like the Draw system. It makes you work for your magic and means that even at endgame you think before you start throwing about your fight ending spells. If you cast it, you will need to go find somewhere to acquire it again.
I like the Junction system even more. This adds another layer to your magic management. Sure you could cast that Ultima spell on that Ruby Dragon, but wouldn't you be better off gathering 100 of them and equipping it to your elemental defense? Making you absorb practically everything?
I like the GF's having abilities. If you played smartly, it would cause you to evenly spread GF's around so that all of your main characters would get the most benefit from them... Rather than just giving Squall all of the OP ones.
I like the how the enemies level up with you. Do you want the game to be easy? Well stock up on crazy amounts of magic for junctioning, and then don't grind on anything. You would become omnipotent and the enemies would remain weak.
If you decided to level up quickly, even basic enemies would get considerably nasty spells and abilities.
Most of all though, I loved the story. This is very subjective as everyone has their own preferences, but to me it was just so epic. It has so many developments and plot twists I would pay very good money to see it done as a 3 part movie, or even as a 700 page novel.
It goes from a skilled fighter doing a seemingly small mission - All the way to trying to save the world from the tyrannical rule of a sorceress attempting to cause all sorts of chaos from the future.
In short, I really enjoyed it.