Wow, so many to choose from.
I should probably just link you here:
Except that in his quest to be clever, he actually condemns a few cool monster (occasionally for really crappy art--another frequently encountered problem in the early days of the game.)
But there are lots (and LOTS) of D&D monsters that will never, ever darken the table of any game I would ever run. But picking my absolute least favorite from among them? The lowest of the low? That's a hard one.
I think I'm going to go with the giff. Space hippopotamus people who wear banana republic generalissimo uniforms and monocles. Whoever came up with that travesty should retreat from the public eye in shame and never show his face again.