Never thought much about it before. D&D has produced some pretty bad monsters over the years.
But my vote goes toward Kender. Man I hate Kender. Do I have to go into the reasons?
Sure, Kender aren't technically monsters--but any creature that's steals from my character gives up their right to be called a "character race"--especially in Dragonlance when said creature steals my character's sword of dragonslaying. You kinda need those in Dragolance.
My character ended up burning down half of a Kender village because of that theft.