I think IX was probably one of the most overlooked. I think maybe because of the game design (as far as graphics) as well with the character design. Even then, though it's a great game. I actually like the setting that took place. It just seems in the shadow's of past FFs. The storyline was great, the characters were well developed, and it was just an enjoyable game. One of my fave FFs thats for sure =P.
I love how VII isn't even in the discussion. Guess everyone agrees that is most likely the one FF that everyone has heard about. Personally, I think VIII is the most overlooked of them all. It's constantly in VII's shadow and most people I know haven't bothered with it at all. I'm glad one of my friends made me play this one first, because I probably wouldn't have played it otherwise.
FFVI and FFXI are definatly overlooked and underated.........FFXI is barely known.......but FFVI i think gets my vote......I believe this could of been the best FF only if they had done this in 3D better graphics.......the story is amazing and the vast amount of character also makes that even better. Even though it is known it deserves much more credit not just for the story but for the music and gameplay as well.