What NPC do your Players Hate

As the title says what character do you absolutely despise. A character that turns the lowliest wizard into a 20th level Barbarian the moment they show up.

For me I have an illusionist that has been harassing the party, stocked up with glyphs of warding and contingencies, designed to be the biggest thorn in the parties side. After faking their death they go off to trick the party into killing one of their beloved NPCs aswell as many other shenanigans. The pure paranoia of how they could be anyone has lead multiple party members to get blindsight or truesight just to use against them, aswell as the party vowing to disintegrate and imprison their soul once they get their hands on them

So what's your most hated character? Vent posts from players are welcome aswell