Network wasn't underrated but has felt more relevant every decade but has been talked about less and less.

Showgirls tells a compelling story about poverty, fame, power, misogyny, and abuse. This film is severely underrated but also it is forgotten by most people who do not go looking for the lowest rated movies to watch ironically.

The second season of Ghost in the Shell: Stand Alone Complex was really good and I find the story centered around a refugee crisis to be a much more compelling one. I feel like this season gets talked about less than the first and the movies, which are generally seen as masterpieces.

You didn't list books but I wanted to mention that Player Piano is one of the most precient books I have read and was written back in the 50s. Vonnegut is of course a well known and regarded writer but you'll rarely see his first novel topping lists even among just his own works, and so it doesn't get read and discussed as much as it should.