Not the one that started it all, but the defining movie in the genre. The story of poor Regan and the foul-mouthed demon that takes over her body is must-see viewing for any horror fan. The movie, like most movies based around an exorcism, examines faith versus psychology, and all these years later it still does it the best. Though the special effects aren’t as cutting edge, the scene where she crawls down the stairs backwards gets me every time.