I hereby declare the Tympole (right) to be the worst Pokémon design ever. Or, at least the worst Pokémon design of which I'm aware.
On the other hand, two of my favorite designs of this generation thus far are the two-by-four toting Timburr and the zebra-based Blitzle. (I also have a bit of a soft spot for Sewaddle.)
To those of you currently playing through Pokémon Black or White: Which designs are your favorites and least favorites so far?
I'm currently about nine hours into the game at this point, by the way. I just chased Team Plasma through Pinwheel Forrest and into Castelia City.