Readers of The Book Magazine ranked Rowling ahead of literary heavyweights including Ian McEwan, Salman Rushdie, Harold Pinter and A.S. Byatt.
Rowling, who is writing the seventh and final Potter book, received almost three times as many votes as fantasy writer Terry Pratchett, who is No. 2 on the list.
The magazine's online poll allowed readers to choose from a list of 50 authors, or to add their own suggestion. The top 10 consists of Rowling, Pratchett, McEwan, Rushdie, Kazuo Ishiguro, Philip Pullman, Pinter, Nick Hornby, Byatt and, in a 10th-place tie, Jonathan Coe and John Le Carre.