Honourable Mentions
Neil Cross
Episodes: The Rings Of Akhaten (Season 7), Hide (Season 7) While "The Rings of Akhaten" is definitely a bad episode, the Doctor's speech to the god is regarded in the Doctor Who fandom as one of Eleven's defining moments. Also, while "Hide" is not a great episode, it has enough potential and decent scares to be considered passable, which is good by Season 7 standards.
Matthew Graham
Episodes: Fear Her (Season 2), The Rebel Flesh/The Almost People (Season 6) "Fear Her" isn't great, but the ending with the Doctor carrying the Olympic torch is a wonderful image for every Whovian. "The Rebel Flesh" was incredibly slow and boring for build-up, but "The Almost People" made up for it with the ganger Doctor and one of the best twist endings I've ever seen in television.
Gareth Roberts
Episodes: The Shakespeare Code (Season 3), The Unicorn And The Wasp (Season 4), Planet Of The Dead (Easter Special), The Lodger (Season 5), Closing Time (Season 6) "The Shakespeare Code" and "The Unicorn and the Wasp" are enjoyable for true William Shakespeare and Agatha Christie fans, but otherwise the cheesy over-acting by the witches in the former episode and the incredibly odd solution to the mystery in the latter can sometimes overshadow the merits of these episodes. "Planet of the Dead" was just god-awful. Luckily, Roberts redeems himself with the introduction of Craig in "The Lodger" and continues with him and Stormageddon, Dark Lord of All in "Closing Time." Now, with those out the way, onto the 'winners'...