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.
The bad guy that is omniscient and omnipresent. Everywhere you go, oops! There’s the bad guy and he totally kicks your ass and ruins your plans.
We call it Neganing. He’s the reason I eventually stopped watching the Walking Dead.
Or like Sylar (from Heroes), where the writers find a baddie they just love too much to kill so the whole show becomes about them.