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.
One shot can definitely instantly kill you. Recently killed things don’t just lay there. They kick, and thrash, and shit, and piss, and it’s incredibly gruesome to watch. That’s probably why they don’t show realistic deaths.