Without going into off model fan-art territory I'd say that Amethyst from Steven Universe, Kimmy from Sym-Bionic Titan and Donna from the Cleveland Show are most likely to win a twerking contest as they have canonically tweked on screen, Flame Princess and Garnet are likely to have Twerking Skills due to their general personalities and careers.
Twerking is a dance that takes rhythm and athletics to properly do, having a giant ass isn't all you need, in fact it can make things harder.
People with super powers such as Helen Parr and Lord Dominator have the physical capability to do it but they most likely don't have the practice.Though someone like She-Hulk, Carol Kincaid and Raven (at least TTG Raven) could pick it up easily
Ancient Beings like Hekapoo, Princess Bubblegum and Rose Quartz have likely picked up twerking or general booty shaking over their long lives.
Animal Characters like Fang from Primal and Horses from Horse Show would likely have an easier time due to their anatomy, but it would be more fair to put them in a completely different bracket.
Cartoon Milfs wouldn't be twerking champions since they are likely not practicing or raised in an environment with any penchant for the skill, the ass is still fat though.
Comedic effect is a factor as well, that may play a part, as a cartoon milf like Dexter's Mom or Debbie Turnbull having skills or natural talent would gross out or embarrass their Children and be hilarious.
For Males it's much simpler, Dick Grayson, Peter Parker and Stan Smith win First, Second and Third Respectively.
