I'll go first. Mine is the instant knockout drug. Like Dexter's intramuscular injection that causes someone to immediately lose consciousness. Or in the movie Split where there's the aerosol spray in your face that makes you instantly unconscious. Or pretty much any time someone uses chloroform.
Explosive decompression in space. It seems to always last forever, suck EVERYTHING out, even if it's a tiny hole through which a giant xenomorph is liquified. The delta P is like one atmosphere, pathetic really.
Then there's noise in space.