The Funhouse (1981)
First, let me say I'm not a fan of slasher films at all. Freddy and Michael and Jason do nothing for me. I find the violence nihilistic, trite, and boring.
That being said, I could see the lasting impact and appreciate the jump scares and chase scenes of the original Halloween (1978). But instead, I picked this little oddity for two reasons:
1) I like monster movies, because they evoke sympathy for both the victims, and (if they're really done well) for the monster, whose monstrosity is not of its own choosing and not under its control (and may even be questionable whether it's a "monstrosity" at all). For this, Funhouse stands out. The teens aren't obnoxious as in many other entries in the slasher genre, and by the end, I really felt sorry for the inbred mutant monster (especially when he got caught in the big gears, I was really crying for him to get out and get free from all the shit that had been dumped on him his whole life).
2)This movie really reminisces about old fashioned carnivals and freak shows, rather than all the plastic, Disney/Sony/Viacom crap our kids are now enthralled by. I'm not going to get too misty eyed over them, but I do think our kids are missing something natural and normal (ironically) in that all they see is antiseptic, safe entertainment, and are kept from seeing bearded ladies, lobster boys, 1000 lb hogs, and dead babies in gallon mason jars. An old fashioned carnival was sexual, dirty, forbidden, mysterious, dangerous - and this film brings that out well, with poignancy as well as disgust. I applaud it for that.