One of my most hilarious moments as a DM was when I ran a group of 8-10th level guys who were rather cocky. At one point they engaged a small group of kobolds and stomped them flat, then proceeded to engage in gratuitous high-fives and self-congratulations.
Alas, they had eliminated the point men of a 1500 member kobold regiment, who proceeded to swarm them with a ruthless human wave attack....or would that be kobold wave? ;-)
When they all woke up hog-tied and gagged in the kobold prison pens, they were a bit more respectful of the little guys.
So I guess my favorite low level monster is not a monster per se, but rather a horde of anything. Hordes are great :-P