Can't think of an overrated FF since I like them all, but as a consequence, I can think of more underrated FF's, mainly FF VIII. This game was plain awsome, the storyline was badass, the intro video and the videos overall were the best for PSX (and it has the best intro ever in a RPG IMO ) the battle system was real fun, the characters were all pretty cool, and it's the only FF I have ever tried to beat for a second time
Then comes FFX. Everyone rants for it's linearity and because Tidus is gay. But they forget the awsome storyline (although a sad fact is that the end was plain stupid and sad), always ignore that it has probably the best battle system on a RPG, outstanding graphics, awsome music (oh, forgot that about FF8, the music is just AMAZING)
That's all that comes to my mind right now.