Aesthetically, my favorite is definitely the Suriname Horned Frog, particularly the green ones.

Scientifically, my favorite is Pedostibes hosii, the Asian yellow-spotted jumping toad (or tree toad). I badly want some for one of my experiments, which deals with the rate of evolution of muscle properties, because they represent a powerful jumper that evolved from a group that's generally pretty terrible at jumping (toads).