Many many great horror films exist...

But I saw The Exorcist as a child. I was maybe 12. It was the early 80s. It was on TV. It was on a 14 inch TV. It was on broadcast TV. It was edited for television. It was in black and white (the TV, not the movie). It scared the shit out of me.

Many a movie has given me a great jump out of my skin moment, but that movie made me quiver with real dread.

I contend that The Exorcist may be the greatest horror film ever made.

On a side note, there no longer exists any 70mm prints of The Exorcist. The last one that ever did exist was destroyed in the early 90s. As luck would have it, I saw that print of the film projected for the very last time, making me (and 50 odd other people) the last people to ever see The Exorcist in 70mm. Not that I want a cookie or anything, but it's kinda cool to know that I saw it that way for the last time.

On an even more unrelated note, I saw a 70mm print of Apocalypse Now that had no credits or title cards whatsoever because it was loaned to the theater I saw it in from Francis Coppola's personal library. Pretty cool. Not nearly as cool as it would have been if Coppola himself came with the film and shared wine from his vineyard... but still, it was my good luck to have stumbled into the opportunity to see it in a public screening. Why bring it up? Well, as a kid, that cow slaughter scene gave me a bit of a shiver too.

I was a very bad boy and managed to sneak some TV when I shouldn't have.

[url=http://en.wikipedia.org/wiki/Puppet_Master_(film)]This[/url] was on and it pretty much scarred me for life, I should never have seen it, and it was my own damn fault.

I'd never seen anything so awful.

Totally agree with you . It makes me sad that all the whippersnappers today actually laugh at the movie

This scene, from Fire in the Sky.

It's still gives me the creeps. Oh God, I shouldn't have watched this again. Oh God.

Watched Stephen Kings "IT" when I was 10, it took me about 5 years until I was able to walk across a Manhole cover (Hope this is the correct term? *gg*)

Incredible Melting Man

http://www.youtube.com/watch?v=a0UOcRB9kkw

I watched it when I was very young. I remember the movie till now.
And also stuff like Child's Play. And some Hong Kong horror movies.

For me, it was this scene (2:46)

http://www.youtube.com/watch?v=ZuLkQMQBZA4
My adulthood is scarred.

http://www.youtube.com/watch?v=UzoPUHKZTNM

and
http://www.youtube.com/watch?v=vdUkPu_rXo0

Both movies made me so afraid of clowns.

This fella gave me many, many nightmares. (Not Benny Hill, the other one.)

http://www.youtube.com/watch?v=XcsK-Ck43LU

And this from Don't Look Now

***HUGE SPOILER IF YOU HAVE NEVER SEEN THE FILM****

http://www.youtube.com/watch?v=g-J9z5cJEGc

Optimus Prime dying in Transformers: The Movie. The only thing that made me sadder is watching my sister die in much the same manner though she didn't give ME the matrix of leadership!

Its really did break me up as a kid and when I learned the truth that his death was a simple matter of the toy having reached saturation and being difficult to sell, that fuelled my hatred of uncaring arsehole corporations

The original Wishmaster film. The most unsettling parts of it for me had nothing to do with the gore or rubber faces. It was the possessed guy's calm but stark raving mad attitude, to my childish mind I could only think that he did in fact act like the devil -- who else would have that happily knowing, "DARE me motherfucker" craziness? Still think that man was a phenomenal actor.

http://www.youtube.com/watch?v=wUnMF7dV86k
Oh yes, they float. They all float down here. You'll float, too!

Off the top of my head let me see...

The shower scene in IT (tons of movies tried to make me terrified of baths growing up. IT, Psycho, the Shining...)

The MEAT scene in Poltergeist (urrrrrrrrgh scarred me so much :P)

As much of a Pink Floyd fan as I am, the whole Another Brick in the Wall pt 2 scene from the Wall. Fucking masks and meat grinder!

Most of the Omen 1 and 2 really messed me up.

There are a bunch more but those are the ones that stick out in my mind.

We stayed up often to scare the crap out of ourselves.

Nightmare on Elm Street and the Friday the 13th movies were always the biggest scares for me.

It's the ones that creep you out in adolescence (and sometimes even adulthood) that are really creepy. Luckily, I wasn't traumatized by any of them until I was a teen. :D

I saw Event Horizon at 3am when I was maybe 15 or 16. Upon a rewatch, not so scary. But at 3am alone, wicked. And of course IT has left me terrified of clowns.

Every time I wake up around 3am still to this day, I think of the Exorcism of Emily Rose.