This is really hard! Anyway here's mine...
1. Steven Moffat
2. Russell T Davies
3. Richard Curtis
4. Paul Cornell
5. Robert Shearman
6. Matt Jones
7. Toby Whithouse
8. Keith Temple
9. James Moran
10. Gareth Roberts
11. Helen Raynor
12. Mark Gatiss
13. Simon Nye
14. Tom MacRae
15. Stephen Greenhorn
16 Matthew Graham
17. Chris Chibnall
To be honest both Moffat and especially RTD are up there by weight of numbers and it's really hard to judge based on a load of episodes, especially hard to untangle the good from the bad with RTD (and in some cases Moffat). I think I've done this as a list of 2, followed by a list of 15 if that makes sense!
It's hard when people have only written one episode, much easier with people like Roberts, Chibnall, Gatiss, Rayor etc who've written multiple episodes and multiple stories. Chibnall's had three bits of the cherry and I haven't especially liked any of them, hence why he's bottom. Gatiss has had three shots as well but at least one of them was v good, with Raynor however poor Daleks was, I liked the Sontarans. As for Roberts, well people slag him off but, Unicorn and the Wasp excepted, I quite like his episodes, fluff, but entertaining fluff. The list is perhaps a trifle unfair, I think most people accept that Graham is a much better writer than Fear Her suggests, and I'm really hoping his series 6 episode will prove this. I didn't like Amy's Choice but I blame that more on the direction than Nye's script, and this is where it gets very tough when people have only written one episode. If we were taking other non Who work into account both Greenhorn and Gatiss would be higher...