I like lang's scorpion from "origami insects and their kin" a lot and foldet it a lot of times. but its hard to say if that is my favorite model... but its surely on the top ten list.
As I note in another topic, I have not a lot of time, so my favorite models cannot be complex model, but, in contrary, very easy and fast to fold models. Up to 7-10 minutes. F.e. rhinoceros of Vojta Drnek