I'd definitely have to say the derp Zaphao. I tend to like the Zaphao's in general, but the derp's give them a personality that I don't see in the normal artwork. Plus, the stages are basically my kittens moods throughout the day; conked out as I go to morning classes, playful/annoyed that we left once he's awake (I know, because my living room is destroyed when I get back), then a cuddly sweetheart once we're home.
My favorite derp is veram! I love this creatu, especially the calico color! I find it funny how the veram looks so nice and has a halo and wings. As the creatu grows up, the creatu grows up (like a teen) and gets into trouble and is very crazy and looks out of control (like a demon). I guess I like this creatu because I can relate to it and how people grow up and they change in their lifetime! :)