The Amityville snorer. I think this movie really benefited from all the media attention the story got.
Giving it a rewatch, I wasn't impressed. I think the most suspenseful scene was when they were looking for the lost cash for the caterer. When money is already tight and you lose a lot of it like that, it is indeed terrifying.
Movies like The Haunting (1963) and Burnt Offerings did this kind of subtle, psychological sort of haunted house tale much better.