This is pretty much impossible as there have been so many good writers in the history of Who.
I have never subscribed to the fashionable fandom trashing of Russell T Davies. He brought back my favourite programme and made it cool again. He gave my daughter the chance to enjoy it. For that I am extremely grateful. Oh, and Stolen Earth and Journey's End were brilliant. He deserves lots of credit.
Likewise Steven Moffat with his ever-so-complicated season long story arcs - heaven knows what he's up to with Miss Clara Oswin Oswald - and stories like Blink that make you wonder what the hell you just watched but when you've worked it out it's all very logical. He loses points for not quite getting the effect of long term kidnapping and having her baby stolen would have on a young woman.
Ben Aaronovitch wrote two of my favourite stories featuring the Seventh Doctor - Remembrance of the Daleks and Battlefield. In the former, the moment when Ace is horrified by the "no coloureds" sign in the B and B is a reminder of a time that's long gone, thankfully. It was a good touch to add in.
Robert Holmes was a huge influence on the series in my early years of watching it so he has to be in there.
Even if you have never watched Doctor Who, you are likely to know what a Dalek is. That's a testament to the work of Terry Nation who not only created them, but also wrote The Survivors which I was probably too young to watch at the time but I enjoyed anyway and created Blake's Seven.
And then there's Helen Raynor, the first woman to write on the modern series. She wrote the two parters involving the daleks in Manhattan and the Sontarans in seasons 3 and 4 respectively.
Terrance Dicks wrote the very first episode of Doctor Who I properly and deliberately watched - the first story with Tom Baker, Robot. And he's written for 8 televised doctors which is pretty impressive.
Tom MacRae wrote one of the best stories of the new series - The girl who waited - which features an older Amy. The story stands up as a good work in itself, but Karen Gillan playing both Amys definitely enhanced the production.
I would never have thought that Chris Chibnall would have managed to show his face in place like this. I will never forgive him for Cyberwoman and Countrycide in Torchwood, but he's kind of redeemed himself but his contributions to Doctor Who in 2012 were pretty good. Not only did he give us dinosaurs on a spaceship, but he gave us Brian Williams as well and it's a pity he didn't get to film his PS to Amy and Rory's story.
Given that I'm such a bit softie, I loved Richard Curtis's Vincent and the Doctor and its sensitive exploration of depression and mental illness. Of course it was going to be a bit schmaltzy - but it wouldn't have been Curtis any other way.
It would be so wrong not to mention Neil Gaiman, too. A writer of his quality was always going to do good things with a Who script and The Doctor's Wife was inspired.
But who to pick as my favourite? I promise you this was not where this was going when I started to write but for sheer imagination and diversity of good stuff over a long time, I'm going to have to go with Moffat. Ok,so he has a bit of a thing for people jumping off buildings and for flying aquarian mammals, but he has built probably the best set of characters in the new series from Madam Vastra to the Ponds to River Song to our intriguing new companion.