Rampage, it's such a good popcorn flick. It's so over the top that I wouldn't blame you for writing it off without watching it, but give it a chance, it's ultimate action silliness.
Cabin in the woods, its a top 2 horror satire, but you may have heard of that one
Tucker and Dale vs evil, maybe watch this one before cabin in the woods, it's the best horror satire.
Galaxy quest, easily the best star trek movie ever made, one of the top 5 movies ever made.
I'm not sure how many of these are hidden gems, it depends on your viewing habits (not many people would consider horror to contain much good cinema)