Krug from The Last House on the Left is my all-time favorite horror movie villain. The character is completely sadistic. From killing a girl and staying at her parents' home to manipulating his own son into shooting himself, he's a very twisted individual. The fact that the actor who played Krug also wrote and performed the majority of the movie's (strangely upbeat) soundtrack adds an unsettling feeling to the movie. The actress that played Mari Collingwood has stated that she was legitimately frightened by David Hess (Krug) during filming of the movie, which goes to show how convincing he was in the role.
The whole movie is up on youtube, ignore the arab subtitles.