I particlularly love the headless human torso with the beholder face!
Also, this is probably the most ridiculous demon ever conceived, the sibriex:
Have a read of the article, then come back here – I have a confession to make. I've used a lot of these stupid monsters. In fact, last night, I used the forsaken shell of a firbolg against the Juma Gang. This was actually the second time I've used this monster - back in the days of the 3.x Freeport Campaign, I used both the forsaken shell and the bhut in the same adventure. I'm going to blame Dungeon magazine for this – they appeared in a cool adventure I ran set in the a monastery called the Winding Way and which featured a number of unusual undead, many from Libris Mortis.
I've also used the petal (annoying faeries), book swarms that inflicted deadly paper cuts on the Luminous Heroes of the Ironcrags, dabus (Jared is absolutely right about their language - luckily kb98 was never cruel enough to me to ask them that many questions when we were playing Planescape) and the ridiculous pseudo-undead from the 1e MM2. I must also confess to using the DM grudge-monster, the nilbog, at least once but in my defense (a) I was probably about 15 and (b) it was in White Dwarf's Top 10 Fiend Factory monsters.
I've never used a flail snail or a flumph though ;)
What's the stupidest monster you've used?