I think Amityville Horror was pretty scarry for me. I know the Excorsist in its days was considered really scarry. I remember when my dad saw it, they handed out barf bags at the movie entrance, and he was really nervous, but when he said he saw it, he laughed!!
I think it is considered really scarry because it is based on religion, and the fear of being posessed by the devil himself scares the catholic based relgion or anyone that has strong religious beliefs.
I myself like the ghost scarry movies, not the blood and gore kind.