It's a pity you only listed FFVI-FFX, 'cause I say FFIV would make the best movie or movie series.

And if you're worried about length, you've got to keep in mind that most of the actual game consists of fighting battles, leveling up, talking to unimportant NPC's, going on unimportant side-quests, and messing around with that game's particular battle system, all of which wouldn't belong in a movie. I'd say the average FF is at most 25% meaningful plot and storyline.