Dredd 3D

Not mentioned once in this thread, is another prove how underappreciated this action flick is.

The violence, the gore, the visual effects. In the R rated department i would rate it higher than Deadpool. With Karl Urban (Eomer - LotR) as Dredd and Lena Headey (Cercei Lannister - GoT) as the antagonistic gang leader MaMa this 2012 action fest was widley ignored and was a box office flop (41m at a budget of 45m). Undeserved.

https://www.youtube.com/watch?v=8W0mvOsCiiQ

https://www.youtube.com/watch?v=IQJH-zUYOoc

Good thread. It has compelled me to created an account because I wanted to add two unappreciated gems that I never see anyone mention.

The 1st is "Stranger Than Fiction" (2006).
This movie is (arguably) one of Will Ferrell's best performances; and Emma Thompson turns in a respectable job, as well. It's about a man who begins to realize he's a character in a depressed author's book when he begins to hear her narrations:
https://www.youtube.com/watch?v=PZpg8VII7es

The 2nd is "The City of Lost Children" (1995).
I had a co-worker recommend this movie. It stars Ron Perlman (long before he gained fame with Hellboy and Sons of Anarchy) as a carnival strong man out to save children kidnapped by an evil genius. Very surreal imagery; well done steampunk:
https://www.youtube.com/watch?v=Pa7oVPru4J8

They Live and The Hidden are great movies from the 80s, that I think that many are unaware of/haven't seen yet.

I don't know if it's the best underappreciated movie of all time but it's definitely really good. Some people have probably seen the remake  (Let me in) but I think the original is better. 

https://www.youtube.com/watch?v=ICp4g9p_rgo

the greasy strangler

Galaxy Quest

Basically the best Star Trek Movie there is - just that it isn't Star Trek

Some underappreciated movies to watch:
I guess they're a bit in the realm of weird cult movies, but if you're open to being flabbergasted by some truly great 'hidden gems' of cinema (to use a tired phrase), give these a try if you haven't already:

Brazil (Terry Gilliam): https://www.youtube.com/watch?v=ZKPFC8DA9_8
(No matter how appreciated this classic is, it'll always still be underappreciated. A visual masterpiece, thought provoking, exciting, super funny. A total rollercoaster ride of fantastical film making)

Martin (George Romero): https://www.youtube.com/watch?v=4SwXSiGpCxc
(George Romero recently died so it feels appropriate to point out a relatively unknown movie that's imho perhaps his best work, or at least up there. There are no zombies here, but there are vampires... maybe. Smart, atmospheric, realistic in tone but totally offbeat at the same time)

The Reflecting Skin (Philip Ridley): https://www.youtube.com/watch?v=gxlnDRqPUXE
(Beautiful and very unsettling. I guess people who appreciate David Lynch will totally dig this little unknown gem too. A fantastic example how the concept of the "Unreliable Narrator" could get totally under your skin)

Exorcist III (William Peter Blatty): https://www.youtube.com/watch?v=uyg8UxnRyQQ
(William Peter Blatty also recently died. So here's a sequel to The Exorcist that's both deeply unsettling and suspenseful, but also has some incredibly witty writing that is often hilarious, *and* has THE best jump-scare of all time? Written by the writer of The Exorcist *and* was a writer of Pink Panther? Not a perfect movie, but you've *got* to see this one)

Kung pow enter the fist.

Last action hero

Primer is great. Pi and Memento also great movies.