Tier 1: IV, VI, X
IV ushered in the golden age of SNES FF, VI perfected it. X is just a damn good compromise between production value and good gameplay and I love the spherogrid
Tier 2: V, VII, IX
Games are in here for very different reasons, V has best gameplay, IX is most charming, VII has a cool world and characters. All of them excel in one area and are worse in others.
Tier 3: III, X-2, XII,
III is the first FF I would call a proper FF, outclassed by later titles. XII has the amazing gambit system. X-2 has hot chicks and a pretty good combat system with atb and dress spheres but is utter shite story wise.
Tier 4: I, VII:R, XIII-2
FF I did not age well, but I yet have to play the NES version with spell slots and no automatic targeting. I suppose they made this worse with updates. XIII-2 is the best of the Fabula Nova Crystallis. VIII:R manages to capture the character very well and has fun combat but not much else.
Tier 5: II, VIII, XIII
FF II is a bad SaGa game, not a Final Fantasy. Cool ideas and I am thankfull it spawned SaGa but the execution is very rough. VIII is absolutely ruined by drawing and enemy leveling, has some cool ideas and the best minigame tho. XIII is just boring, a game that has literally nothing besides combat.
Tier 6:
IV The after Years, XIII-3, XV
lol
Edit: MMOs not rated since I cant compare a game that takes 500 hours to one that takes 50. I liked XIV ARR tho.
Spinoffs not rated because there is a billion of them