I previously had V pinned as the most underrated game, but it turns out many people really love it and it's almost never hated on.
Meanwhile, XIII is vastly discriminated against. It's easily the most underrated game. It's not even that bad, it has it's problems like every FF, but they aren't as bad as many claim them to be.
I'd say XII comes after XIII. XII had great characters and a pretty good story. It deserves it's spot as the second most underrated FF.
As for which game is the most overrated, it's actually quite obvious.
X is incredibly overrated. It's easily the most overrated game of the series and it's followed closely by VI. Everyone and their mother raves about X and VI these days, even though they're not all that astounding like many claim them to be.
Before people come in and say that VII is the most overrated, I'll ask you to think again.
VII gets hated on a ton these days. It gets less respect than Rodney Dangerfield, so you know it's not overrated. I think it's about middle ground, leaning slightly towards overrated perhaps.
Once upon a time it was true that VII was the most overrated, but now that everyone claims VI and X to be the saviors of FF, it's no longer true.
I think rational people will see the logic in my arguments. You might not necessarily agree, and that's fine, but the logic behind my reasoning can't be ignored.
A fan of a badly received game is angsty against the more beloved ones. Nothing new really. --- "I REMEMBER MARKING EVERY VICTIM WITH ACUTE PRECISION, I REMEMBER EVERY TIME I HOLD YOU, MY BLUNT COMPANION. WHEN I REMEMBER, I DISMEMBER!"