J.K. Rowling is a brilliant author and is the best in her genre. However, she is not the best author in the world. No one doubts Rowling's storytelling ability--she is on par with Dickens and Austen in terms of having characters move through her stories in complicated ways. The "Harry Potter" novels are masterworks of children's literature that adults enjoyed reading as well. There are plenty of other talented writers who are just as worthy of being called "the best" just like Rowling. Stephen King, Terry Pratchett, Dan Brown, Amy Tan, John Grisham and a host of others are equally as impressive as Rowling.
The best authors are those that write meaningful books, that will last a life time. Anyone can write a fantastical story, this is just a function of how loopy someone is. On the other hand, to write something that makes sense in the real world and gets the attention of the mainstream is noteworthy. There are several authors that come to mind, too many to list, however suffice it to say that anyone who writes fiction, is not the best writer, unless that fantasy is abstractly based on reality and the real world.