Let's try and narrow this one down. There's a show called Crooked House by Mark Gatiss which contains one really scary moment, but we haven't covered it on British Invaders so I don't want to go in to too much detail just yet.
I considered both Blink and The Stone Tape. I actually watched Blink again last night and it is still a superbly crafted bit of television that manages to be exciting, funny and scary. The Stone Tape may not have aged terribly well but I remember seeing it in the 1970s and being really disturbed by it. All I could really recall about it was a terrible sense of something old and malevolent inhabiting the walls of the building being investigated by the scientists. It's not quite so scary now but at the time it was a pretty good scare.
In the end I have gone for a piece of experimental television that seems to divide people. Either you find it really scary or really dull. Jonathan Miller's film Whistle and I'll come to you is a black and white adaptation of M.R. James' ghost story Oh, whistle, and I'll come to you, my lad made for the BBC arts programme Omnibus in 1968.
Michael Hordern plays a rather fusty academic on a walking holiday in Norfolk. He finds an old bone whistle near a graveyard and blows it. After that ... Well, not very much happens, but what does is really rather disturbing. Jonathan Miller changed the tone of James' story and made it a contemplation of loneliness and sexual repression. There is very little dialogue in the film, Hordern's character often communicates by just grunts or whispers or muttered repetitions of other people's statements. The bleak Norfolk coast line looks suitably chilling in black and white and the whole thing has a strange and disturbing dream-like quality.
It really creeped me out when I first saw it and still does. Interestingly it has recently been remade with John Hurt in the main role. The 2010 version is more about bereavement and loss and has some good moments but I still prefer the 1968 version. You can find it on youtube and you can listen to our review of it on British Invaders.