I've beaten all of the main title single player games (plus quite a few others which I won't comment on).
I - It is what it is. I enjoyed it, but it's too much of a grind and doesn't have much for story for it to be one of the best in the series.
II - Loved it. Level up system is confusing, but it's best to avoid grinding if you can and is better experienced not abusing self attacks. I like the system, oh well. Has some of the most memorable music in the series and one of the better stories. PSP version made some great adds that explain the world's back story, too, while also including the great addition from Dawn of Souls.
III - Enjoyable game, but characters aren't as well done as in II. Party chat is the best development in the DS version. My favourite class system in the series, too, I don't understand the complaints about the 'learning curve' and needing to stick choices out. Felt like a nice challenge to make things work and have to think about your party choices before trying any combo. Loved Cloud of Darkness for some reason, albeit a bit out of the blue. If there was one big change I would make, it would be adding a save point at the point of no return. That last dungeon is cruel if you loos to the last boss.
IV - Great game, though Kain starts to drag on the story and some of the characters get annoying. Feels a little more directed than the first III, though, which I don't like. ...not sure it's true, either, though.
V - Ugh, find a mode of transportation, find the one path it can take, wreck it, repeat. Terrible characters, weak villain, silliness abound, but at least it got better as the game progressed. Class system was a joke compared to III's. You could instant switch any character to suit your needs, use an ability from mastered classes... at the final boss I think I had all four of my characters in brand new classes. There just wasn't any need for foresight, here, bleh and cheap.
VI - Loved it. the game just hits a fantastic spot when you get to the open world part, but this game was great start to finish. Final boss is probably my most memorable in the series. Ultros was a little off putting, but I think more fond of him now, and will probably appreciate him when I replay this game.
VII - Disc 1: Ugh, so much annoying content, some of the worst in the series. Disc 2&3: Great time, some of the best in the series. Yeah, that sums it up, game didn't live up to the hype but I can see why so many people love it. For me, it's just not one of the great ones, but for all those that aren't, it leads the way because so much of it was a great game.
VIII - Ugh, so many annoying characters, a terrible story, and an awful implementation for battle. It says something that I though the 'no encounter' buff was a fabulous addition to the game since you could get it so early. Battles were for farming, otherwise you're just making the game harder by leveling up your enemies, silliness. I felt stuck, damned if I battle normal and damned if I don't, I just saw it as taking the lesser of two poor choices. At least it had some of the best music in the series.
IX - Great game. Too bad some of it got a little annoying, and I'd have liked it a little more open. The core characters, Zidane, Dagger, Steiner, and Vivi were all fantastic.
X - Ugh. Annoying characters, terrible world design, linear and random battles mesh for a dreadful walk, eliminated active battles, stupid sphere grid they pretend is open, and a superb story somehow mixed into some downright awful elements like blitzball. I had an ok time with it, but I don't think I will ever understand the love this gets as a game.
XII - Loved it. Felt so open even in the times it wasn't so much so, loved the side quests, loved the characters and their development (yes, even Vaan and Penelo). Gambits were a lot of fun to play with, but also optional so I don't get the complaints about them. The one drawback is length, I'd love to replay this game but it's a huge undertaking for a story based game.
XIII - Well, they were clearly trying to repeat X, but it went a little better this time. Battles were active again, they didn't try to hide how directed leveling up is, linear path was so much more tolerable with visible enemies, and, finally, something like Gran Pulse with some real exploration showed up. Still has many of the faults of X, but Gran Pulse ensured this game was one I ultimately enjoyed, overall.