I guess it would depend on what you're asking about when you say "underrated". If you're going for entries that are great but don't get any recognition from the majority of the fanbase, I'd say it"s FFIII and FFV. If you're going for games that are not well received but are actually pretty good, I'd say FFII, FFVIII, and FFXII. If we're expanding this to the whole FF franchise, I'd lump in the FFTactics Advance series and FF Crystal Chronicles in as well. Hell I'd even throw in Final Fantasy: The Spirits Within, because it is far from being a terrible movie and it always gets me in the mood to play FFVII and IX after watching it.