The worst is Piplup. It's evolution is great, but it is bad. Why? Because it just looks like that.
This has to be said. The worst design has to be that new Ice Pokemon. The one based on an ice Cream. I mean, what the hell were they thinking!
Trivia time: It's evolution line was also the only line designed by an American.
American stereotype time: That is why then. Americans only think of food. 
Americans these days...and the last ice cream evolution looks like an ice cream version of Weezing...