When I was little, I could be rather scared of aliens. Somehow I still grew up loving SciFi. Go figure.
I was scared of ET when I saw him for the first time. Learned to stand his wrinkly face during the movie, but he was still creepy in the first few scenes.
I had the same reaction a while ago seeing that first video of an alien in Signs. The "yikes, jump backwards, want to flee" reaction.
Weirdly enough, I enjoyed the Alien trilogy as a horror flick, but without the same scary jumpiness.

Though I think number one on my creepiness list are still the ones that hide looking like perfectly normal people (or kids) and only later turn creepy

Which TV/Movie/Literature aliens scare you most?

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Clive Barker's Cenobites (Hellraiser - 1987).

I actually had to walk out of the theater, couldn't take it. And I mean just after Frank was ripped apart [ ] -- what, just 5 minutes into the film?

Finally, 8 years later, I was able to watch the film via VHS. Am curious what the remake will "look like," but in my mind nothing will ever top the original Hellraiser; was still rather young at the time.

I don't like scary movies, so I avoid horror movies pretending they are SF. So I've probably missed out on a lot of candidates.

For me there were two particularly notable ones. Back in my childhood, I remember watching some 1950s SF movie about some space alien on Earth that was hiding in a cave and killing people at night. For most of the movie, all you saw of it were two glowing eyes in the dark. That one gave me nightmares for weeks, imagining all sorts of things hiding in the dark.

Number two has to be the larva from Alien and the scene from the first movie where you see it for the first time (in the lunch room). I remember seeing it for the first time when the movie first came out, and not knowing what to expect. More than the scariness of the creature, was the surprise. There were people in the theatre that were freaky out from that scene.

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That brings to mind an old film which sister and I watched a few times on late-night TV. It involved these locust-type aliens which invaded Earth. Their eggs were discovered in honeycomb-type fashion in walls. Later the entire city went up in flames -- and at film's end the flames formed a sinister devil's face.

Scared us. But we always had to see that film again, if it were re-aired.

Have tried to discover film's name over the years, with no success.

That sounds like Quatermass and the Pit; an excellent film. Aka Five Million Years to Earth for some reason.

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The xenomorph from Alien, hands down. It's progressively less scary in the sequels.
I was a bit creeped out by some Dr. Who monsters. The Ogrey from the Key to Time storyline scared me when I first saw it. And the krynoid, until it grew bigger than a house. Not the Daleks or Cybermen though.
From a literary aspect the colour out of space is something I find scary - probably Lovecraft's best story. And the bathtub woman in The Shining freaks me out. I find hotel bathrooms are a little creepy to this day because of that book. She's not really an alien though.

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The Id Monster from Forbidden Planet, hands down.

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The Id monster never scared me. Too obviously animated, perhaps. Still a nifty idea, but not really scary.

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I think Heinlein's Puppetmasters are as scary as they get (I know, they were probably a metaphor for totalitarian governments).

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Everyone reacts differently, but when you are seven years old in a dark theater...

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Though not aliens, really, the flying monkeys form the original 'Wizard of Oz' were the most frightening/fascinating things I'd seen as a child. Had nightmares about them, but couldn't get over the morbid curioisity.

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Signs' aliens hands-down. Scary, but not because of anything about the alien's design. The suspensful buildup to the climax made it work for me. Something about it hit close to home as I recalled helping my parents make preparations for approaching hurricanes. The feeling you get as you're making your final preparations and the minutes until the storm arrives are ticking away is indescribable, but watching Signs brought that feeling right back into my throat. By the time the final "shocker" hit the screen I literally jumped in my seat - something no other movie has been able to make me do. Two years after Signs came out our house was hit with 3 hurricanes, one right after the other. I lost that sense of dread because of how routine it became in the course of that year, but I can still look back at Signs' aliens and remember what it used to feel like growing up.

Yeah. That thing was stomach churning. Ugh. Still "gets" me everytime I see it.

This may sound lame, but the fake aliens in Scooby-Doo and the Alien Invaders scared me when I was little. The real aliens at the end of the movie didn't scare me, though, because they were nice. But the faking pulled off by the bad guys was so seamless that I really did believe that evil aliens were invading!

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The one in K-Pax. Kevin Spacey is just creepy.

Yes! I still remember the first time I saw that. I was young (that helps increase the "scare" factor) and when I saw it, I was very tired. Things always seem more intense for me when I'm half awake. It made enough of an impression on me that I can remember a number of things about that day (otherwise, it probably would have faded away).

And, yes, it was called "Five Million Years to Earth" in the U.S. It's probably because Quatermass is largely unknown here, and they thought that title would get more attention.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

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The Blob (50's version). Cherry Jell-O that eats you!

Fred

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Dragons Domain Space 1999

http://www.youtube.com/watch?v=7tbXhu09m5s

Kinda like a PZ Myers in space...

Certainly freaked me out as a kid, & so did the Sontarans, though Mike has a very good point with the Id Monster..

But for sheer 'I don't want to be on the same planet as that' how about

The Thing

Edit: & now I see the Literary scary thread..

Another vote for the xenomorph.

The Thing, from The Thing (1982)

Not a movie, TV or from literature, but The Flood from the first Halo video game.

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Reminds me of when I saw the Outer Limits episode, "The Zanti Misfits" when I was about eight years old. A big old B&W TV had been put in the basement, and I could go down there to watch stuff more interesting than what was on the family room TV. So, I remember watching this alone, in a largely unfinished basement with large dark corners and occasional creaking noises. And there were these alien bugs that would sneak up on people . . .

Today it looks silly (here are some scenes from the show), but I do remember that I couldn't watch that show through - at least, not that time.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

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Aliens have never really bothered me. Neither have most ghost story type shows. There was the occasional creepy episode on TV that really wouldn't have been scary if I hadn't been 8 (Fantasy Island used to creep me out a bit), but never to the point they kept me awake or gave me bad dreams.

No. The scariest movie creatures I have ever seen, and which still affects me to this day, are the cats from Lady and the Tramp. Freaking baby eating Siamese cats. I was 5, and we had a Siamese that hated me. To this day, I'm not comfortable around one. I almost think I'd walk into a room with an agitated rottweiller or pitbull before one with a calm (plotting) Siamese cat.

I worked with a guy who was in his 30s and still couldn't watch the Wizard of Oz all the way through due to the flying monkeys.

Another guy around the same age was still scared of Witchiepoo from H.R. Puffenstuff.

The Girlfriend's Dad is still freaked out by one scene in The Birds.

To me, the scariest alien would the one you never see. A devastated town or planet, bits of tech that can't possibly exist, but that's it. No physical evidence at all of what may have actually done it. If you know it's a bipedal reptilian, then you know what to look for. If you only know it wants you dead, then every sound, shadow, and smell is a potential threat.

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