I'll say that I absolutely love Halo 2, and play it more than I should, but I still believe it's overrated. It's hailed as the best FPS ever by many people, but it's nothing revolutionary--it's just designed very well.
I don't know if saying these games are "underrated," especially, but Eternal Darkness and Mario Kart: Double Dash are both incredibly fun titles that don't often get much attention. There are tons of other games I could comment on, but this'll do.