9. Sleepaway Camp - Angela Is A Man
Essentially, Sleepaway Camp is the diet coke version of Friday the 13th for the most part. It's not as big or famous and it's not nearly as good, but it's defining twist is something that has elevated the movie into infamy in the years since its release. Portraying a camp plagued by a mysterious serial killer, kids keep dying gruesome deaths whilst they're supposed to be rowing and playing volleyball - being stung to death by a swarm of riled up bees is hardly a fun summer camp activity, after all.
After plenty of unfortunate 'accidents', it's revealed that the central innocent protagonist Angela is actually a boy who's been raised as a girl, murdering everyone that has wronged them in a psychotic rage after a lifetime of abuse at the hands of their aunt. Whilst the revelation is heavily dated now, at the time it was one of the most surprising twists a horror movie could spurt out, with Angela's surprise killer instincts carefully hidden away until the very last moment.
Pamela Voorhees's revelation was just as good, but then Friday the 13th isn't a medioce movie.