I hate to break it to you but according to IMDb David Crane hadn't directed a single episode of Friends. He was the producer of the series.
Directors of television shows are far less recognizable in terms of name recognition as their film counterparts.
But if you're simply using producer and director as interchangeable terms, this question would still be a tough one. I pay far more attention to the world of film directors and who directs each individual film and film franchise then I do about TV episodes let alone TV series.
Off the top of my head:
Joss Whedon for Firefly and Dollhouse;
Alan Ball for Six Feet Under;