Unown. It's based the alphabet for crying out loud. At least Voltorb is a ball that can explode. What can Unown do? Hidden Power? Hah. Despite how he looks I still accept these as Pokemon. But worst designed though, but kinda creative.
EDIT: anyone not saying Unown, especially saying common crybaby complaints like Vanniluxe or Trubbish, aren't real Pokemon fans.
I don't get the hate for Trubbish. He was clearly supposed to be a parallel for Grimmer and is basically the same thing.. except instead of being toxic sludge, he's actual rubbish. Considering that, I think the design is quite cute and it's hilarious in its absurdity.
Gigalith on the other hand completely fails design wise. I think it's an idea that could have been great, but feels like they just said "screw it, that's good enough".
I also think most of the recent legendaries are horrible, with the "pantomime" horse trio taking the cake for worst Pokemon designs ever.