That's a harder one for me, I would have to say my favorite is Eevee, like someone else said earlier. He is a great and reasonable size, and he looks so cool by any gender's side. A great pokemon for the front of your party. My least favorite is Charizard. Not that I have anything against him, he's awesome and can learn great moves. It's just he looks really strange by a female sprite, especially a Shiny Charizard, and looks decent by a male, except for the fact that it's wingspan is so small and un-realistic.
I guess Charizard might be cooler if he was a midget. :D
~Caxno