While going through the Who list, I paused at "School Reunion," but carried on because it was only one episode and it wasn't an absolute favorite. And then I checked the episode writers for Being Human and he wrote four of the six episodes and is the overall guiding hand of that show. I wasn't obsessed with the show, but I thought it was very well-written and had a wonderful overall theme that played out beautifully over the course of the six episodes. And thinking back on SR, he did a wonderful job selling so many moments in that episode and there was some fantastic dialogue that just hummed with power and emotion. So, him first.
Sure, sure, he only wrote three episodes for Doctor Who, and two of them were a two-parter, but they were some of *the* finest episodes the show has ever produced. "Father's Day" is arguably the best episode in series one. (Personally, I'd put it second-best, but it's so close up there and on so many favorite's list, I can't say that it *isn't* arguably the best.) The characterization, the heart, the storytelling, just all of it was amazing. As for his two-parter, "Human Nature/The Family of Blood," it's not one of my favorites but that's just because I wasn't a fan of the actress' portrayal of Joan, but that didn't take away from the magnificent storytelling and dialogue. And true, only two episodes, but he carried the arc fully from moment one to the last without losing any of that heart and power. Supposedly, Cornell is who Russell T Davies wanted to take over. I so would have been happy if he had.
Peter J. Hammond
This is based solely on one episode, but I adored it so: "Small Worlds" on Torchwood. My favorite episode prior to Children of Earth, this one is. Lovely, gorgeous episode. Seriously, I was near tears at the end there. It was just sad and beautiful and poignant and I LOVED seeing Jack deal with the consequences and how his "companions" were unable to understand his choice. It was so awesome seeing a "human" point of view with what the Doctor does all the time. And Hammond got that and portrayed that so beautifully; I can only imagine what magic he could wring from the history of Who.
He wrote "The Unquiet Dead." Period. He also wrote "The Idiot's Lantern" which I liked -- the rewatch may make me realize I like it more, but bottom-line for me is ... he wrote "The Unquiet Dead." If you have some time to kill, check out my rewatch-review (I've GOT to get back on those!) of the episode ... you'll see just how much I adored, adored the writing of this episode. Just fantastic!
Finally, Shearman of "Dalek" fame. Alas, the only episode he wrote for the show. It's such a shame because it's a brilliant episode that combined the history of Classic Who with New Who in the best fashion the new program has done, in my opinion. His writing for the Doctor and Rose was superb, and like Davies' he has a deft hand in creating smaller roles -- that only need great actors to make them bigger.