I went with Silence of the Lambs.
One of the key themes in this poll - with the noted exception of The Exorcist - is the creation of a legendary villain. Well, not Freddy, Jason, Michael, Johnny (Shining), Jaws, Leatherface, Ghostface, etc. can even approach the legendary status of Hannibal Lecter.
I always bring up this stat - to my knowledge, only two films in history have ever swept the Academy Awards top 5 trophies - Best Picture, Actor, Actress, Director, and Screenplay. Those films are One Flew Over the Cuckoo's Nest and Silence of the Lambs.
Horror is a genre often panned by critics and beloved by audiences, but Silence captured them both. The honest cinematography and the development of the Clarice Starling character are both magnificent, but nothing beats the existance of the painfull evil - but undeniably likable - Hannibal Lecter character. Anthony Hopkins brought this character to life in a way few actors have ever done, and considering he wasn't the first to play the role (1986's Manhunter saw Brian Cox in the role, admirably).