I've said this many times before, but I believe Impidimp could've done better if it's design had a less flashy color palette, because that, in my opinion is what makes it the most disturbing. Another one I didn't like much was Zacian, it doesn't have a bad design, but the way the sword theme was implemented in it was very poor imo. There's also Legohead Dreadnaw, which I don't dislike, but I don't like either.
I wish they'd scrap the water/fire/grass type starting tradition. Imagine how interesting having to pick something else like psychic, fighting or dark would be.
I don't think we'll ever see that disappear, from the main series titles anyway. They just work decently for getting new players into the type advantage gameplay mechanic, which is less obvious for Psychic/Fighting/Dark to get across.
Currently, I don't dislike any of the new Pokemon, but my least favourite would probably be Meltan. I think its design is a little weird, and it was in the Let's Go games while the cross-gen evos were left out.
Duraludon. Maybe it will involve into something more impressive but I don't like its design and nothing about it screams "dragon" to me. I don't see why Game Freak made Tyranitar its natural enemy when it's at a 4x weakness disadvantage.