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.
Thanos was a great villain by this standard. He had lived through the suffering and collapse of his homeworld due to overpopulation, so his motivations actually made sense.