I think no one has mentioned “the man from earth”, it is a great movie that got a lot of success thanks to piracy, enough that it allowed it to even get a sequel (I haven’t watched it it seems to be far less recommended).
I recommend to just watch it without looking too much at the theme or plot. I’ll just say that it’s a movie with a lot of conversations and basically no action at all. You could compare it in that sense to the classic “12 angry men”, not in plot or theme, but style, mostly something that happens between a small cast of actors through dialogue.
Edit: and just a few minutes before me someone did actually share it…
Most likely I’m too specific about it, but I see quite some shows mentioned that I’d not really consider underrated or unrecognised. Firefly is one of my all time favourites but not really underrated or unrecognised. Or Fringe, possibly not even From (of which I’ll just say I very much hope they have an ending planned because I don’t want it to end lost like Lost).
Perfect examples of underrated or unrecognised shows are Galavant and Utopia, mentioned by others, they are brilliant.
In a similar direction I’d say Dead Set, a UK show that really surprised me ages ago but never see mentioned. https://imdb.com/title/tt1285482/
Maybe I should include also Dr Horrible’s Sing Along Blog, but that may be a lot less underrated and it’s a minishow only.