OOM – they were billed the best two children’s authors of 20th century. by I don’t remember which publication.
JKR invented precious few concepts in her books. 95% are regular magic/myths etc that she took further. Animals, spells, etc – its cute and imaginative how she does it, but…
Dahl simply came up with brilliantly new ideas. In a fraction of the words, he creates a world that draws you in. And all the more vividly, because like a good author, he makes you a partner in the process, forces you to imagine with him, and doesn’t just feed it to you, like JKR.