I can't decide between posting my favorite or posting the feature I want most sooo...

I'll just make a list.

FFVI: The story was awesome, and also we need more fun stuff in the story...like the opera
FFVII: GOLDEN SAUCER!
FFVIII: Triple Triad
FFIX: Learning skills from equipment, Chocobo Hot & Cold, Trance
FFX: Sphere Grid style for traits, so you get a sphere grid and some spheres from quests and what have you, then you can choose traits to augment your class/job skills to make yourself more unique.
FFXI: If not the Tactics job system, this job system
FFXII: Gambits trolololo. No, just kidding. For real I would love Hunts/Marks...Faction leve NMs are sort of like hunts but I'd prefer if they were non-repeatable and could include cameos (Gilgamesh hunts too good!) Also the bestiary from the diary.
FFXIII: Uh...I didn't really like this game, but content-specific jump
FFTactics: Job system