Actually, I think Douglas Adams (Hitchhiker's Guide to the Galaxy) had the best one. I forget the name of the creature, but it was apparently a flesh-rending horror. However, it was so stupid that it would think that if you couldn't see it, it couldn't see you. The key to escaping a horrible death was to cover your eyes if you made spotted it.
Anyhow, that theme brings us to an article about some of the dumbest monsters in Dungeons and Dragons. I'm inclined to agree, although I'm thinking that having a room of death with just the floor, ceiling, and walls is not enough.
Hat tip to Shamus, who generated a great discussion, including the guy who made one of the stupid monsters. Fantastic.