Scythe de Zaran
I think I played some of the older ones, but I don't remember if I finished all of them. I remember the one with Terra, Locke, Edgar, Sabin... I think that was the 3rd one. Might have touched the 1st and 5th one before. Saw the 7th, and haven't done anything past that.
Yeah, this is bound to confuse you even more, but that FFIII you played... that was really FFVI. Only three Nintendo-era Final Fantasies were released in America, so they changed the numerals for them, but went back to normal when FFVII hit the Playstation.
I'll just summarize some of the plots so you can know for sure which ones you have played. I will not include any handheld or side-games, only the main series of console games.
WARNING! There are spoilers, though small ones for the most part. Don' read if you intend on playing them one day and don't want it to be spoiled.
FFI: The four warriors of light travel around the world to defeat the four elemental fiends and restore light to the Crystals, facing the embodiment of Discord, the God of it, Chaos in the end.
FFII (Not released in America originally): Firion an his allies fight to defeat the evil empire, eventually fighting and defeating the emperor himself.
FFIII (Not released in America originally): A new group of Warriors of Light gather together and try to protect the Crystals of Light and the Crystals of Darkness, eventually facing the powerful Cloud of Darkness.
FFIV (FFII in the first American release): Cecil Harvey, a Dark Knight of Baron, goes on a journey of redemption, becoming a Paladin and fighting to reclaim the captured Crystals, fighting against his brother, Golbez, and the one manipulating him, Zeromus.
FFV (Not released in America originally): Bartz and his companions try in vain to prevent the Crystals from shattering, and eventually come into a conflict with the entity that the shattering releases, ExDeath.
FFVI (FFIII in America originally): Terra, Locke, Sabin, Edgar, and a group of twelve other heroes all gather together to fight and defeat the source of their unhappiness, the evil ruler Kefka. To aid them, they use the Crystals given to them by the Espers, powerful summon spirits that Terra is the child of, making her an Esper as well.
FFVII: Cloud. Sephiroth. Jenova. Shinra Corporation. Meteor. If you need to know anymore, then I'll go into more depth.
FFVIII: Squall and his fellow mercenaries are raised from a young age to fight and defeat Sorceresses. They end up going to the future to defeat the powerful sorceress that had manipulated Sorceresses in the past to try to eliminate said mercenaries that were supposedly meant to kill her one day.
FFIX: Zidane Tribal and his friends fight to stop the conquest of the Alexandrian Kingdom, eventually uncovering a plot by a world in another dimension to absorb their own world by causing an endless flow of death and interrupting the flow of souls. They fight against the embodiment of the death in order to prove the meaning of the Will to Live and prevent the end of everything.
FFX: Tidus is swept 1000 years into the future and protects the summoner Yuna in her journey to master the Final Aeon and defeat a monolith of destruction named Sin, made from the soul of his own father. Forsaking the Final Aeon, which only perpetuates the rebirth of Sin, they fight it themselves and defeat the ancient summoner that forever summons it, Yu Yevon.
FFX-2: Yuna returns on a journey to find spheres, hoping to find some clue as to the whereabouts of Tidus, who vanished at the end of X. In the end, she fights a giant machina that uses the power of the Farplane to try to destroy everything. This act convinces the souls of the departed Aeons to bring back Tidus to live with Yuna once more.
FFXI: Don't care. It's an MMO, so it doesn't count.
xp FFXII: Vaan and a group of rag-tags fight to save the world of Ivalice from the empire that threatens to conquer all. (Can't remember the name for the life of me right now.) On their journey, they also learn the secrets of the Mist, Magicite, and Nethicite, all powerful forces that influence the course of their adventure.
FFXIII: I haven't learned much about this one really, but I plan to eventually. All I know is that they have their Eidolons forcibly implanted in them by another Eidolon called Anima.