Overrated: FFVII, definitively. I don't hate the game, but as it was already pointed out by Ouch!, it's not the end-all-be-all of RPGs. It's an okay game. It was innovative, yes, but like all the other titles in the series it has its flaws.

Underrated: FFT. Or if you want to stick to the "main" titles, FFI-III.