Which NPC is the most dangerous for its level?

Pound for pound, which monster punches hardest? The roster runs from fodder (Slime) up through Sentinels, Guardians, Rangers, Catchers, Creepers, Lurchers, Wisps, Spitters, Thornbacks and Scorchers to the genuinely scary stuff — Behemoth, Broodmother, Vampiric Ooze, Minelayer, Voidcaller. My vote goes to the Minelayer, because the danger isn't where the monster is — it's where it has been. But I've also watched a Broodmother turn a clean fight into a swarm nightmare in seconds. What's your "never again" NPC, and what's the tactic that finally made it manageable?