Never in my adult life have I been freightened by anything in film or TV. Maybe as a child I was, but I don't recall an instance off hand.
Single most so moment? That's ridiculous -- that's like asking what the single best score is.
Some scary scenes I can think or, just for the sheer surprise, execution and believability:
That fantastic seen in "Psycho" when the cop is going up the stairs and he's rushed and slashed across the face and falls down the stairs. That look on his face, the wound across it, and the great was it was filmed from his face as he fell down, with score, is gripping.
"Timeline" was a particularly unmemorable and nothing-special film, but that one moment when the likable good guy is suddenly run threw with a sword, that was a shock; you'd expect him to have stuck around.
The old episodes of "The Twilight Zone" ("The Midnight Sun"), with the break down and panic of dying, followed by the twist ending with it's own shock, well -- shocking.
That moment when Buffy finds the body in "The Body" episode of "Buffy the Vamprie Slayer". Quiet, out of nowhere, no scoring, just a shock.
Special mention to "Jaws". That ariel shot as jaws comes up to the man with his mouth wide open (followed by the leg scene), is just a gasp scene. It's especially a great scene compared to a lot of today's shock scenes: there's no dramatic punch (especially a loud punch just to make you jump); it's not suddenly, you see it coming; you feel it, you're not detached; and it's that kind of scene where I bet members of the audience were, bakc when it originally was shown, telling him to get out of the way -- you can't do anything about it... in the best possible kind of way.
Of course that moment with the captured men in "The Great Escape".
There's this scene, I don't remember the name of the film (I saw it on a video of horrible film deaths on Youtube), where this guy is in a closet, trying to surprise his friend or was hiding from his friend, either way his friend didn't know he was in there and opened the closet and yelled and jumped back and drew out his gun and blew a hole in the guy's head, splattering blood everywhere -- shocking.
"I'm a leaf on the wind ... watch how I fl--". No. Just kidding. ;-)
Some scenes become less scary with repeated viewings, but the first time I saw Psycho, I was terrified. The shower scene and the detective getting stabbed at the top of the stairs horrified me. Of course, the music added to the terror.
ALIEN scared me many times. I agree with Heath about the Kotto/Cartwright scene near the end. Horrifying. Music, editing, glimpses of the monster's tail pulling Cartwright towards it were terrifying. The scariest aspect of that scene was Ripley hearing Cartwright's panting and final screams in her headphones. Nothing has scared me more.
There are a couple that spring to mind, but may I please advise people who haven't seen these movies to not watch the clip on youtube but actually look for the movie or just don't watch it at all. You should not be experiencing these moments from watching a youtube clip, it completely demolishes IMO the impact that moment has if watched outside of the context of the movie! I know Montana means well but I simply cringe from seeing pivotal moments of a movie displayed in this manner.
The scene at the nurse's station in Exorcist 3. Beautifully executed.
Everything about EXORCIST III is scary. The long scene down the corridor while the nurse goes in and out of rooms is the scariest scene in a motion picture, to me.
So glad to see THE MUMMY cited. (See the post I just left on the "UNIVERSAL MONSTERS" thread.)
As to THE HAUNTING, this is not widely known, (I don't think it is even mentioned in the book about Wise's films), but at a seminar a few years before his passing, Mr. Wise revealed that what we see on the actors' faces in that door scene and others are their responses to pre-recorded sound effects which Wise had prepared to help them have something besides their imagination to react to.
My own favorite fright scene? As usual, I refuse to play by the rules and limit it to only one. Just a few, off the top of my head...
PSYCHO shower. The first time I saw it in 1960, I couldn't even look at it, so all I saw was my fingers in front of my eyes. But you all know what I heard.
Several moments in the British DEAD OF NIGHT, including the mirror strangulation scene, and the climactic nightmare.
Likewise, several moments in the "Pigeons From Hell" installment of the THRILLER! TV series (from a story by Robert E Howard, directed by John Newland, hauntingly scored by Morton Stevens.)
And of course, Preacher's cellar showdown with the children, and the subsequent chase to the river, in THE NIGHT OF THE HUNTER.
Aye, that Exorcist 3 scene is spooky, but the other, more drawn out bit, where the nurse is going about her duties and when the audience can't be any more relaxed, the guy with the big swordy thing shoots out from a doorway to attack her! Lots of other creepy bits in that film too, just simple scenes like leaves blowing down the street at night...such a feeling of dread.
The scene at the nurse's station in Exorcist 3. Beautifully executed.
Everything about EXORCIST III is scary. The long scene down the corridor while the nurse goes in and out of rooms is the scariest scene in a motion picture, to me.
Yes, though I was even more freaked out by the shears sequence later on (especially that effect that highlight the movement) and the climax of the movie also had my heart stop!
The scene at the nurse's station in Exorcist 3. Beautifully executed.
Everything about EXORCIST III is scary. The long scene down the corridor while the nurse goes in and out of rooms is the scariest scene in a motion picture, to me.
I agree. EXORCIST III is intense and scary. I found a fanedit recently which restores, insofar as is possible, the director's original intent. That means cutting out the priest played by Nicol Williamson and the exorcism which filmed and inserted after the completed film had been screened. It's infinitely more effective without them. The understatement of the whole thing really works.
Invasion of the Body Snatchers (1956)--When Miles kisses Becky in the cave and realizes she has changed and she yells out, "He's here."
As classic as the original is, the 1978 Snatchers remake has the most shuddery ending I have EVER seen in a horror movies...still makes me jump to this day. Everything about that movie is masterfully unsettling.
Also The Descent, where we get a night vision camera POV shot when one of the women is screaming, "IS ANYONE THERRRRRRRRRRRRRRRRRRRE?!" as the camera keeps whipping back and fourth, and when it comes back to the screaming woman, there's a creature standing RIGHT. BEHIND. HER. I exclaimed out loud when I saw that moment for the first time in a theater.
With a nod to Francis (hey, Brussels!), I will say here: DO NOT VIEW THIS CLIP UNLESS YOU'VE SEEN 'Trilogy of Terror'. I may be stretching the term of 'Frightening' here with this clip from this early 1970's made-for-tv film, but after I first saw it, I kept looking under the couch and in the corners of the room.
Everything about that movie is masterfully unsettling. 
