IMO, it's FF8! :D. FF7 is almost the same cause there are still few unanswered questions left in FF7 but apart from FF8 (duh!). Other FF games are okay, nothing to debate about, although I'm not sure about FF9.
Whatever SquareSoft did to FF8 did a very good job, they let us discover and debate about the game for A LONG time which is quite... clever, what can I say?
Should all FF games be more debating like FF8?