This is really hard. Probably the original Dracula with Bela Lugosi on the telly as a child.
Got obsessed with Christopher Lee as an early teen though.
As a 'grown up' in the movies The Omen and Jaws - horror is a strange category really, perhaps they both fit.
It might have been Night of the Demon though. My dad made me watch it with him, I was a teenager but young, I think - I have never, ever forgotten it. One of the creepiest, most haunting films I have ever seen.