Wow, I don't...
Final Fantasy IX? Xenoblade? Regardless whether they didn't reach the sales targets set by forum goes, I can tell you that between here and reddit it's hard to go a week without seeing these games mentioned, so whatever under-ratedness they had has long been washed away by cult status (edit: Shadow of the Colossus too, for that matter).
My pick: Gregory Horror Show. Haven't yet met anyone else who played it. Fascinating pseudo-survival horror game.