Copypasta from another forum I visit.
FFI - I played the remake on the GBA first and later the true original via virtualnes.com. The true original has a poor magic system and is kind of unintuitive, but the GBA remake, which retains all the core gameplay but makes it more accessible to new users, is an extremely good game. It's even more impressive when you think about how revolutionary it was for the time, with the class systems, a good story, and a great deal of customization. I played through this game over 9000 times with all sorts of party combinations, early class changes, low level class changes, even a party of 4 Black Mages (facing Chaos was very difficult with this configuration btw, especially when he healed himself lol). Overall a great game and definitely in my top 4.
FFII - In contrast to FFI, this game is a giant steaming pile of **** by comparison. It's definitely the worst game in the series, by far. I'm not even going to talk about it anymore, it's best forgotten.
FFIII - I played the DS version and enjoyed it a great deal. The job system in the game is a little dissappointing compared to more modern RPGs and there are some unbalanced classes (Ninjas, Sages, Geomancers, etc.), but I guess for it's time it was very good. What really irks me about it is the save system and how long the intervals between save points are, especially considering how difficult the game can be at times. All in all, though, it's still a good game by today's standards and it's definitely in the better half of FF games.
FFIV - I've played the GBA and DS remakes, and each has their pros and cons. The GBA version isn't as difficult and doesn't subject you to poor voice acting, but the DS version's augments are pretty cool. Anyway, as for the game itself, it's great. Until I played VI, it was my favorite game in the series. I like the fact that you have 5 party members and I find it odd that no other game in the series did this. I also liked the story at the time, even though when I look back on it logically there were too many deus ex machina moments lol. It was still a really great game though and I loved it, and it's definitely still among my top favorite FF games.
FFV - I played the GBA remake but didn't really get to a whole lot of the extra content since I spent so much time on the game itself. This is basically a game for people who are hardcore about character customization. This was fine for me, but the fact that the story was kind of bland compared to the rest of the series made it seem like the game didn't have too much to offer. The difficulty curve also ramped up near the end, so I had to spend more time grinding for job mastery, which was tedious. There are 2 main things I like about this game. The first is the music. FFV has one of the best soundtracks in the series in my opinion. The second is Gilgamesh. For those of you who have not played FFV, Gilgamesh is the biggest mother****ing baller in the game, by far. He has the best theme music, he gets defeated by you 3 times, thrown into an interdimensional rift, and then defeated again before he dies, has a sweet-*** pimp hound, and the best dialogue of any non-Kefka character in the series. Gilgamesh alone makes FFV a good game.
FFVI - My favorite in the series. What can I really say about it that can't already be said by other FFVI fans? It's utterly fantastic and there wasn't a single moment of it I didn't enjoy.
Before I go on, I haven't played any of the PS1 games for an extended amount of time, so I can only provide brief impressions on a few hours of gameplay.
FFVII - I played up to the point where you have to rescue Tifa from the whorehouse, or whatever it was. It definitely seemed like it could only get better from that point on, but not necessarily like it could become my favorite in the series. Maybe someday I'll go back to it, idk.
FFVIII - I played up to the graduation or acceptance into SeeD or whatever that scene is called. This one I didn't like too much. It seems like they had good intentions with the Draw system, but in execution it's just so tedious. The story also didn't pick up a whole lot and I didn't have a ton of motivation to continue with the plot since I didn't see a goal ahead aside from beating Seifer in a "least likable" competition (yes, I know the plot is more than this). The weapons system I didn't really understand either. FFVIII is not a bad game and I guess I really shouldn't judge it since I haven't played through it all the way, but it's definitely not one of my favorites in the series.
FFIX - I liked this one a lot, and even though I didn't get very far (Garnet's renaming after the first Black Waltz) I still think It's safe to say that this is my favorite of the PS1 games. There's not too much to say about it, it's just a really solid game overall.
FFX - My favorite PS2 generation game and probably in my top 3 overall. I like a lot of things about this game, especially the story, the sphere gird, the weapon customization, the music, and the battle system. I haven't' technically beaten it, (I'm on The Calm Lands at the moment) but it's definitely one of my favorite in the series.
FFXI - I'm surprised nobody has mentioned this yet. I used to play it on the PC for about 6 months and it was pretty good. I guess I can't really compare it to anything since it's so radically different from the other FF games and I haven't played any other MMOs for a significant amount of time (I played WoW on a friend's computer for a few hours once), but it's still pretty solid and I enjoyed my time with it.
FFXII - First I loved it, then I hated it. The battle system is great and the License board is also very good if you know what to get when, but the rest of the game pretty much falls short. There's a lot of grinding you need to do in this game, for exp, money, and LP, which I don't like. Towards the end of the game the hunts are also hard to do since it takes some annoying way to spawn them or whatever (I JUST WANT TO FIGHT GILGAMESH, IS THAT SO MUCH TO ASK!?!). I also couldn't really connect with or like the characters since Vaan is ********, Penelo is a *****, and everyone else speaks formal English which at times can be hard to understand, let alone relate to. The music of the game is also a departure from what I like since Nobuo Uematsu was almost completely absent from it and I think the quality suffered because of this. The setting, the characters, everything about the game just makes it seem like a generic RPG. And in the end, that's all it really is. One of my least favorite FF games. Not nearly as bad as FFII, but still in the bottom 3 or 4.
If I had to rank the ones I've played for a significant amount of time, I'd probably go with this:
VI, X, IV, I, III, V, XII, II
If I had to rank the ones I've played regardless of how long I've played them I'd go with this:
VI, X, IV, I, III, IX, V, VII, XI, VIII, XII, II