1. Paul Cornell
Easily the best candidate for taking over Doctor Who, Paul Cornell has a long and productive history with the show dating back to 1991 when he wrote a novel for the New Adventures Doctor Who series. Since then he's written countless amounts of novels, short stories, comics, audio dramas....you know what, just follow
this link to his entry on the Doctor Who wiki site. Then finally in 2005, he got to write for the revived series. His episode "Father's Day" was a huge success, but was soon eclipsed by his next two episodes: "Human Nature" and "The Family of Blood", a two-parter. It enjoyed massive acclaim, and is remembered as being one of the finest episodes of the revived series.
So, thoughts anyone? Are there any other great writers who would be even greater on Doctor Who? Sound off in the comments below!