Originally Posted by Joe the Rat
The humble Rust Monster - A simple creature that exists solely to put the fear of Og (and Rustoleum) into metal-clad warriors.
Seconded. This is one of THE best monsters. I'd also like to mention Gelatinous Cubes.
There are some that are hilarious, but not really GOOD. Duckbunny, Armadillephant, Giant Space Hamster, really any from here