In order Best - to - Worst
FF7
FF8
FF9
FFX
(Not played much of X-2. X-2 is more like some sort of sugar coated britney pop song video.)
I think Squaresoft went wrong when they didn't bother keeping the tradition going with Materia from FF7. It's always the same when you read reviews - "Squaresoft introduce another brilliant, completely different magic/summon system".
Someone really needs to tell these reviewers that no one wants the stinking new ones, Materia's for keeps.
On a less critical note, I decided to stick FF8 on, havn't played it in quite a few months. The story goes - I bought it when it came out for sale, and played it none stop for ages, then my friend asked about it, so I brought it round and they started a new game. I was in a rush to leave, and out of habit I just hammered the buttons several times to save and saved over my file :/ - 80 hours of gameplay down the pan - had everything as well, all GF's, Final Weps, all but Quistis lvl 100 :/. Quite naturally I was annoyed, who wouldn't be? :whathuh:
So I left it for a while, then came back to it a year or so ago, then left again, after I got up the same previous status I had, minus a few levels, and 2 final wep's.
Finished off getting all the weapons today, all I got to do now is level up a bit and go to Ultemecia and complete it, like 5 years after it was released hehe. (Can't wait to see the ending, heard it is good)
Same with FFX as it happens. I'm trying to max character stats at the moment. Got all celestial weapons, with break damage limit and so on, but need to get break HP/MP armour. Essentially I could complete FFX right now, as most characters do 99,999 damage with each melee attack, (even Yuna :devil: ) and umm, what's his name, Anima can get a good 1.3 mill out of a mob if it has low defence but I find it more fun if I know I can pwn any mob ingame.
Still got the Monster Arena to fill up 100%, as well as Penance to defeat, (without Yojimbo hehe).
I'm not a complete drop out though, I've completed 7 and 9, (7 many, many, many times).
Strongest parts of 7 were it's story, and magic system. Sephiroth and Jenova were so enticing, soundtracks always helped as well.
With 9, it was the emphasis on Fantasy, as a few people have mentioned - Everthing made of wood comes to mind. Was dissapointed with Zidanes limit breaks, or overdrives, or whatever, but the ability system made up for that - I was practically indestructable with Auto-Life/Regen combo.
If anyone wants any help, needs any tips, give me a PM, or you've got something to share, add it here

Wonder if there is anything I
Don't know about FF7 - I hope so. :devil: