What film, show or game that is not necessarily 'underated' didn't have the level of social impact it deserved.
What's the antithesis of Arrested Development, Firefly or The Big Lebowski? Those may never have 'found their audience' but over time seemed to recognised by everyone. What are the deep cuts that you liked but it feels like everyone has completely forgotten they even existed.
I feel like Children of Men (2006) doesn't get much recognition for being a chilling view of a future world or the respect it should with it's amazing long takes and excellent camera work. If possible watch the behind the scenes material about the car rig they built. It's so cool!
It's consistently mentioned for both of those aspects, actually. Like, google pretty much any review of the movie and you're going to see one or both of those things mentioned with almost absolute certainty.