Two oldies that spring to mind. Maybe not underrated but definitely forgotten.
The Snake Pit - early 1950s? - starred Olivia De Havilland as a slightly unstable woman who is interred in an asylum only to deteriorate into a mess at the hands of the asylum's medical staff and doctors. It had such a profound effect at the time that the US Government was forced to implement the biggest mental health reforms in US history.
Little Big Man - early 1970s - starred Dustin Hoffman, who as an infant with his older sister, were the sole survivours of a Chyenne Indian massacre. Hoffman was brought up as an Indian but when captured as a 17 year old, his loyalty is divided between the races for most of his life. I saw this as a 12 year old and it left a huge impression.
Don't forget the craziest movies of all time!
Salo
Sweet Movie
Irreversible
A Serbian Movie
The Human Centipede