"Day 15 - Your favorite horror film involving serial killers"
I take "serial killers" to mean one person kills a whole bunch of people and so with that, I shall go with "Sleepaway Camp"!
Oh "Sleepaway Camp". You are so special to the world. The film tells the story of some kids who lose their dad in a boating accident (not related to "Jaws") and they're forced to live with their fucking crazy ass Aunt Martha. Some years later, the kids are almost teenagers now and Martha sends them to a summer camp.
We focus on Angela, who's very shy and quiet. Everyone at the camp teases her while she just stares at them oddly. Soon, people end up dead or hurt very badly and the camp director wonders if Angela has something to do with it...
This is a movie where the main effect of it is the ending. You watch it and thinking "Well the acting is terrible, it's a standard slasher" then the final scene occurs and you're like "OMG WHAT THE FUCK IS THAT SHIT?!?!" and you go hide in a closet somewhere. Anyway, there are three sequels, which I haven't seen yet, but I'll be getting to those soon.