What you find scary

Specifically in Aliens. What kind of alien scares you the most? The blob like Blob, dissolving people in clear view? The Xenomorphs, bursting out of chests with acid blood? Or perhaps the Thing, the perfect mimic?

Do you remember the 1985 movie LifeForce? I find that concept far more scary, than aliens piping out of chests, or the blob, or the Thing. An alien just like us, except they are more beautiful, and they create desire, lust in us. Yet in the back of your head, you know at some level they simply can't be that beautiful.... But you WANT them....

It's based on The Space Vampires by British science fiction horror author Colin Wilson. The vampires are not our normal blood sucker kind, but rather energy suckers. And they have the ability to take over our bodies and use them for their own purpose. Also the aliens are not what they seem either, they are not the beautiful people they first seem to be, nor are they the giant bat like creatures, they are then thought to be...

The British have always been more open when it comes to sex, while this isn't a porn film, at least when I first saw this movie I remember there being a strong sexual component lurking in the background unstated. This feeling that you'd do anything to have sex, with this creature, you are sure of isn't really human, but you can't help yourself... they have the ability to rob us of our humanity, to throw us back to a more animalistic state.

Then when they suck the energy out of us, it doesn't simply kill you, but it ages you, makes you grow old....

I find scary aliens with the kind of hard-wired neurological activity of insects--they are inexorable, have no particular self-awareness, and cannot be induced to do other than their instinctive behavior by reason or coercion. Imagine going to war against a colony of human-sized ants. It doesn't much matter what they look like; it's the utter lack of "humanity" in them that terrifies, the fact that they are something that we cannot possibly exercise any influence on.

That our simple, cute trash-compacting robots are equipped with industrial-grade cutting lasers that can slice through a refrigerator in under a minute, and can perform similar feats on human tissues and bones. Particularly the fact that these little guys are intended to help us. Also goes for larger, very talkative robots with nuclear-powered lasers on their shoulders. But they aren't really aliens, no. They only act like aliens. But I'm definitely going with the aliens that said they were going to be our friends, but have great capacity to hurt us. Like the ones in the 50's version of The Day The Earth Stood Still. They warned us they could vaporize us all, but chose to just cut off our elelctricity for an hour just to make a point.

To make a truly scary aliens, you must think of something beyond simple disgust and lethality and go into the realms of complete terror. This year I had a team of alien parasite scouts hiding in the forest, so a few of them stole the bodies of small animals and others took the forms of people that recently died to walk again, but the one that scared the most was the one who fashioned a body out of gourds and sticks (but that was personal; his teammates infected and killed a young boy's parents when he was seven and he only barely got away, only to confront the creature again when he was twelve and thought the other was dead). At least these were just the scouts; the super-soldiers would be armed to the teeth and wouldn't know when to die since few weapons could scratch them and even if anything did, they could regenerate.

Also consider the idea of an alien that can use mind tricks to make your worst fears come to life and traps that don't even begin to play fair.

nano bots turning everything into "gray goo"

You want scary? Aliens just like us.

The Zerg.

"Beware the Destroyers. They come by the millions from the Realm of Darkness which extends where no stars shine. For a thousand generations They slumber, lying in wait. Great nations rise and flourish. There is peace and prosperity. Then comes the Dark Times; then They Return. They cull and burn. They are warped, and move beyond the pale, bigger than any man, unnatural births. Every nail, claw-scale and spur, every spike and welt on the hand of those heathen brutes is as barbed steel. It is said there is no honed iron hard enough to pierce Them through, no time proofed blade that can cut Their brutal blood caked claws. Armies are raised and cut down like grasses before a scythe – it is said the armies of Amorah and Suhdom, each ten thousand strong, were swept away between a single rising and setting sun. Heroes come forth and are slaughtered. Lightning and fire rain from the sky and the whole earth trembles. They are as a deluge, a powerful flood that washes away entire mighty nations and empires. The people pray for deliverance from the gods. The gods fight the Destroyers, but their efforts are in vain. Iapetos, Ausare, Ehecatl, Souchang – all perished. The Destroyers are to the gods as the gods are to men and men are to insects: cold and vast and unsympathetic. The wise flee Them; the lucky escape. Fifty score great vessels were launched to seek refuge from Them; only those led by Satyavrata, Utnapishtim, Noach, and Deucalion escaped. The Dark Ones vanquish all before Them. They have come before since before time began. Now They slumber. They will return. Beware the Destroyers.
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Proclaimed again by Her Majesty Victoria, by the Grace of God, of the United Kingdom of Great Britain and Ireland Queen, Defender of the Faith, Empress of India, in the Year of Our Lord Eighteen Hundred Ninety-One."

With thanks from the SCP Foundation.

The scariest sci-fi stories I've encountered:

The Dr. Who episode "Midnight."

Kij Johnson's "Spar" (Likely NSFW, and BIG TRIGGER WARNING).

Alien

What pushes my SF/horror buttons are stories where the creature becomes a catalyst for humans to morally and psychologically self-destruct.

To me, the truly terrible Aliens are the ones beyond our comprehension. Think Lovecraft. Something so alien our minds have to come up with our worst nightmares just to make us from going insane.

For something more corporeal, any Alien thatcan wipe us out. An above poster said the Zerg-look up their Grandaddy, the Tyranids. Or to draw from 40k again, the Necrons/C'tan. Something so bizarre we have to make comparisons to things we know, and even thenthe fact that we are so pitifully outmatched...