"Hybrid" is a term tossed around a lot in the film industry when talking about a person who does multiple roles on a film set. George Clooney, for instance, is no longer a "one-brid"; because he has directed 3 films, he is now an "actor-director", much like Orson Welles or Kevin Costner before him (I'm sure he'd prefer to be paired with the former than the latter….ironic, given my choice of #1 on this list).
This list is about people who were well-known and established as writers, and then began directing. My rules are basically that no writers are allowed who have always been hybrids — if they directed their first screenplay, for instance, or pull a Darren Aronofsky or Paul Thomas Anderson and always direct their own work, then I can't have 'em on here. The main thing is that all of these guys had to have an established career as a writer FIRST, and then moved into directing.
In order of greatness, these are, for me, the best directing jobs of any screenwriter in Hollywood.