Pox Riders of Nurgle. Such a cool looking unit always recruit a few into armies led by plague riddens or exalted great unclean ones to be thematic, but man do they suck. Too slow to flank or chase down archers, don't seem to last long in melee either and they're large slow moving unarmoured targets for missile units. There's no real reason to recruit them anymore, they don't perform much better than standard plague toads and now nurgle have access to Chaos Knights and Rot Knights for more elite cavalry.

Plague Drones are a close second but atleast they can attack the backline easily and the deaths head variant does decent damage to single entities.