If you take into account when each game was released then FFXIII. If you don't then FFXIII is still a valid candidate. The game was inexcusably linear, practically made you play on auto, it had a repetitive structure with almost no diversity in gameplay and many design flaws within what it had, it had nothing of what is needed to craft a world (towns/npcs) and relied on datalogs (external sources) to convey part of it's lore and story, writing was uneven with forced character development for some and downright laughable for others including some minor and major villains... I can keep on forever... Anyway, none of the other FFs have so many flaws on so many key aspects.
By the way, why are there sequels to some of the numbered games listed but not for others?
By the way, why are there sequels to some of the numbered games listed but not for others?