Tough one....i love my endlii bichir, my flagtail, my new 7" african leaf...but i think my favorite, for the moment, is my white calvus.
I don't subscribe to the "rarer is better" mindset so this may seem strange but: Convict Cichlid. Pound for pound (or ounce for ounce) I have never seen another fish with their ferocious disposition. They're cheap, readily available, breed easily, have a great personality, and look fantastic. What else could you want?