I don't think there are any "bad" designs per se, but I definitely think Druddigon is the worst design compared to every other one.

Its colors are super basic, it's super flat for a pokemon with "Rough Skin," its moves are boring, you always find it super late in the games, it's shiny is easily one of the worst...

I mean, I don't hate him, but I think objectively speaking he's the worst design. Compared to Aromatisse, my least favorite Pokemon, he's god-tier at least, but at least she seems like she has some sort of purpose (being inspired by overweight fashion nuts, probably)

If you really want to hear my full thoughts, I can link you to them, but that's the gist.