A series is rarely underrated and for all the screaming we few on this forum that hate Halo there are a legion of loyal fans that made Halo 3 one of the biggest launches in gaming history. I rarely hate games and I don't hate Halo, its an average game, I hate the fans. As a matter of fact that is the reason I hate Final Fantasy so much.
Ah well, on topic, as I said if its a SERIES its most likely not underrated because it sold enough to warrant a fucking sequel.
With that being said, underrated games are Planescape: Torment, Septerra Core, Psychonauts, and Beyond Good & Evil.