My favorite is the flumph!
The second stringers are the Yuan-Ti, Rakshasa, and Nagas. I'm also fond of intelligent apes. They all have a great pulp fantasy Robert Howard exotic feel to them. Furthermore, the greatest aesthetic strength of all of these is in the non-combat arena. It's all about the lost civilizations they form and the forbidden cults that worship them. Just discovering them is half of the story!
My favorite fiends are Juiblex and Mariliths, although I did like the idea of that guy stuck in a block of ice in the BoVD.
Come to think of it, I seem to have a thing for the Indian themed creatures. Never thought of it before...