I'm gonna say Grandia, even though I only played Grandia 3, which happens to be one of my favorite jrpgs ever. Hell, it's one of the best games of the sixth gen, imo. I'd absolutely LOVE to see Grandia IV on gorgeous 1080p on the PS4. Darksiders is also a great game nobody remembers. Still haven't played the sequel though.
When it comes to individual games, The Legend of Dragoon and Twinsen's Odyssey quickly come to mind. The first is one of the best rpgs ever but, despite a strong fanbase, is NEVER mentioned anywhere. It was the best-selling PSOne Classic for a long time, so there's hope.
The latter is a PC game barely anyone knows. It's one of my favorite games ever, I'd really love a sequel.
Rogue Galaxy, Digimon World Dawn, Call of Juarez Bound in Blood and Half-Minute Hero also deserve honorable mentions. And then stuff like Okami and Mark of the Ninja are not underrated, but overlooked
