Maybe not underrated, but Disgaea definitely deserves more attention.
The entire Sonic series also gets way more shit than it deserves. Aside from 06 and Shadow, which I thought wasn't that bad, the series is pretty solid. Sure, I wouldn't call any of the games amazing on the level of something like Mario Galaxy, but it's far FAR from being as bad as many make it out to be. Naturally, I'm referring to the main series games, though even as far as spinoffs go, the only real major offender is Boom.