My personal favorite is still VIII. My only gripes are 1. the game was too easy once you understood the junction system and 2. the characters other than Squall and Rinoa were too flat and static. Squall and Rinoa were fantastic, but the others were just dull.
Second is probably VII. Too linear overall IMO (seriously, if you look back at it, you can foresee 90% of the game as soon as you get out of Midgar) and the characters didn't really change much. Still a lot of fun though, and a compelling story for what it was (linearity doesn't make an epic story suddenly stop being epic altogether).
After that is probably VI...but I need to replay VI to be able to say why I enjoyed it.
Then probably XII....XII is under-appreciated IMO. It doesn't deserve way too much praise, but it was a good game overall. My biggest gripe is just that Vaan and Penelo became pointless almost as soon as Ashe came into the picture, and that eventually there was so much time between plot events (especially if you went exploring) that you would quite literally FORGET what had happened last and why you were going in a particular direction. Then the combat...beyond the early parts of the game, the combat really does play itself, for the most part. Set an attack gambit, heal gambits, resurrect gambits, and run around. That's it. You very rarely cast spells or do anything other than just mash physical attacks. Even in the last boss fight, the only time I stopped spamming physical attacks is when HE BECAME IMMUNE TO THEM, ffs.
Then probably X. X was an absolute blast when playing it...but looking back on it, it's actually pretty mediocre. It's EVEN MORE LINEAR than VII; seriously, other than the Seymour subplot and the two twists near the end (one of which is the ending itself), pretty much everything you're going to do is known once you get to Besaid. Another big problem is that the "end game" (that is, doing the advanced monster arena stuff) is an epic grindfest of doom: firstly, you have to do the grinding just to UNLOCK the things, which is agonizing, and then to do the advanced content you basically have to get 255 across the board and then spam Quick Hit until things die. At least in fights like the Omega Weapon fight in FFVIII you actually had to be somewhat inventive instead of being able to quite simply spend hours waiting.
Then probably IX. I don't remember enough of this to pin down what I didn't like about it. I just remember I didn't. Maybe it's because I was expecting to see FFVII but better (back then I had only played VIII and VII, in that order) and didn't, I dunno.
Then X-2. X-2...fairly good for what it was really, but not deserving of its title by any stretch of the imagination.
Haven't finished V or any of the earlier ones (I played about half of V before I lost my save, and I've barely done anything on the earlier ones) so I won't comment on those.