Tenchu my personal fav and Einhander although I don't think that was 'underrated'.
Some others I will have to think about but there are many gems on that system :)
Current Favorites:
Wii - Muramasa, MH3, Metroid Trilogy, RS2, Okami, RE4, COD, Dead Space, Scarface
PC - L4D, TF2, BFBC2, Killing Floor, Aquaria, Warsow, many UDK indie games
Mods - MechWarrior:Living Legends, Star Wars: Galatic Warfare, PVKII, Goldeneye Source, L4D mods
PS3 - Demon Souls, Way of the Samurai 3
Social Modding Club: ModDb | Steam