I saw OUTITW when I was seven at the cinema but I only ever saw the beginning/half way (used to get taken out if a film was unsuitable, grandparents were cinema inspectors), but I saw the beginning about 3/4 times.
For years the only bit I remembered was the family being massacared, it even became a recurring dream that I have had on and off for about 30 years. It was the unexpectedness, the shot of their feet all lined up, their boots, the table.
I only saw the whole film about five years ago and had no idea it was the same film. So I was totally stunned when my dream started appearing on the screen, it was like watching something you knew really, really well but at the same time it was something that only I should know.
Consequently, it gets my vote because although I spent a childhood watching films in some very, very seedy cinemas throughout southern england, it's the only film that has ever managed to affect me so much that it crossed from being a film into being a memory that's pretty much haunted me since I saw it.
(just goes to underline the importance of film classification)