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.
That can be tricky, though. Done well and you get a villain like Gul Dukat. Done slightly less well, such as with Killmonger and the villains from both Incredibles movies, and you have to make them at least resort to “for the evulz” methods to prevent the audience from sympathizing with them, and even then you may be left with a broken Aesop at the end.