I voted for Mxyzptlk. He's just downright adorable and funny. And I liked having some magic in the show, this was at least good magic and most certainly possible. Okay, maybe I'm biased, since it was so much fun to write a fic around him.
My others votes went to Bernie and Jack. I think it's quite obvious why Bernie, mostly what others already said. And Jack? He's cute!
Too bad he got off the show, he had a nice role, imo.
Saskia