What is the scariest Sci-Fi concept you've seen?

[DSPE]

For me, that might be the Leviathan. It is a galaxy massed... object that is 10 light years in diameter. And it is heading towards us.

"The Thing" from the titular movie "The Thing (1980)" could also be considered quite creepy.

[Worldtree]

The most direct answer: using a direct neural interface to induce a constant state of panic and fear into your own brain. It’s probably quite possible for entire habitats to join in on some kind of Fright Night event..(or someone else doing that)
or as a method of torture and punishment in a less civilized region. A lot can go wrong in a few years in an independent system before enforcement arrives.
Also the thought of how many ways a group of people could be manipulated and their will bent to make a life worse than death if a local terrorist or regime chose to do that. 
Also the thought that ahuman archailects could pretty much rewrite reality to whatever they want and the willpower of anyone won’t matter.

[Worldtree]

Anything like the leviathan is vast and incomprehensible but is almost too big to have feelings about, for me, in the way the death of a star is unstoppable and far away in the future.
And in OA , beings like The Thing could end up being your neighbors in the habitat in a nearby orbit, as long as they’re sophont and you have translation tools/Ontology platforms

[DSPE]

And in OA , beings like The Thing could end up being your neighbors in the habitat in a nearby orbit, as long as they’re sophont and you have translation tools/Ontology platforms

Not if they are constantly subsuming people, like the creature does in the titular movie!

[DSPE]

Also, even if the Leviathan isn't malevolent (we don't know what its nature is), it might just disrupt orbits and smash into entire star systems, even if it doesn't mean to. It's quite massive, after all.