This is my favourite so far:
Shock 1977
http://www.imdb.com/title/tt0075651/
Shock (also known as Schock, and Beyond the Door II) This was Bava's last film before he died of a heart attack in 1980.The movie was actually started by Mario Bava but finished by his son Lamberto Bava. The film starred Daria Nicolodi, John Steiner, and David Colin, Jr.
This film had a very eerie feeling to it. My favourite moment of the film was when
Spoiler...
Marco ran towards his mother, threw open his arms, and transformed into long-dead Carlo.
Elegant, effective, and simply frightening. Gave me a start too. There's such a grace in the way this film is edited together, particularly in the last act. The ending moment has a neat special effect and is crafted well. Very fitting to the movie. It's full of memorable imagery. That, in my mind, is very important part of a horror film.