Although some novelists were excellent screen writers, adapting both their own work, as that of others to the big screen, top original screen play writers are mostly a breed of their own.
(s.a. "Favorite Dozen Bestseller-Authors adapted to Film"
and "Favorite Dozen Top-Playrights adapted to Film")
PS. Admittedly, these lists overlap somewhat as a number of authors should have been included in two or more but weren't simply because I needed the room :-).
Disclaimer: I purposefully named this listing a "Favorite" and not a "Top"-list. I know I have left out a number of top candidates (due to the fact that a dozen = 12, grr) and am afraid I've left out even more authors I never even thought of in the first place.