Well, as many have said I didn't play too much of II because I really didn't get on with the levelling system.
Haven't played I, III, IV, V or any of the online ones… guess I'm not as hardcore as some of you, but out of the ones I have played…
VI: Finished it once, wanted to play it again and get into all the extra stuff like finishing the cultists tower and defeating all the dragons but the second time around I just couldn't maintain my interest. The story is good, but really rather bloated. Especially the second half where you need to track everyone down again. It's padding plain and simple and it really isn't necessary. Shame really because I enjoyed some of the characterisation and I also enjoyed the climax.
VII: Love it to pieces. Everytime I hear any of the music from this game it sends me into waves of nostalgia and I get the overwhelming urge to play it again. I love the environments on this one, especially the various areas in Midgar. The Shinra scene with the blood splattered all over the floor is one of the eeriest video game horror moments ever. An absolute masterclass.
VIII: My first Final Fantasy. I could play this over and over and never get bored of it. Unlike a lot of people I actually enjoy the Junction battle system and it's another game where even a couple of notes from the soundtrack make me want to dive in and play all over again. And Triple Triad is one of the best FF mini games period.
IX: Didn't like it much the first couple of times. After 7 and 8 the medieval dungeons and dragons tone was a bit of a shock to the system. But the characterisation on this one was absolutely brilliant and by the time I was on my second play through I really did care about the characters and what happened to them. And Kuja was absolutely badass, wardrobe notwithstanding.
X: Probably the first Final Fantasy I'll turn to if I fancy a casual run through. I don't even know what it is about this one that I enjoy so much but I really just love playing it. Probably a lot to do with the slower paced battle system giving me more time to think through my moves. The voice acting is terrible and some of the characters are really annoying but none of that stops it from being my most regularly played Final Fantasy.
Really did not like XII at all. Might never play it again. Won't miss it.
XIII: Haven't played it yet. Own it but for some reason haven't actually opened it and taken the plunge. I think I was burnt with XII's newfound emphasis on not being in direct control of the entire party's movements which I did not like. I hear XIII is a little in the same vein as far as the focus on peripheral AI vs controlling all active fighters, but we'll see.
Gee, sorry for going on.