I realize this isn't a universally held opinion, but I think all of his episodes are fine or better. "42" is a nice, tight little thriller, "Hungry Earth"/"Cold Blood" is an effective if not terribly inspired reintroduction of the Silurians that honors a lot of the ideas in their original story, "Dinosaurs on a Spaceship" is an enjoyable if silly romp, and "Power of Three" tries a lot of different things and succeeds in a decent number of them.
I wouldn't consider him in the top tier of Who writers, but while he's never written a classic, he's never written a failure either. He's a solid writer for Who, if not much more than that. And I say that as someone who watched all of his Torchwood stuff and didn't like most of it.