By underrated do we mean lower critical reception eg metascore then it should have had or do we mean not as much sales as it deserved.
I loved Digimon World for the PS1 but the reviews for it, put it as a mediocre game. Not what it deserves.
Also Killer7, No More Heroes 1 & 2, Psychonauts, Beyond Good & Evil, Space Station Silicon Valley, Okami, God Hand, Ico, Jumping Flash!, Body Harvest, Madworld, Alan Wake, Grim Fandango, Red Steel 2, Muramasa, Jet Set Radio Future and Haven: Call of the King.