I've long judged a good back in a back double bi as largely, if not equally, based on good thick lats.
Dorian's lats were largely overpowered by his immense traps. In my opinion, someone like Phil Heath, for as much hate as he gets by most people here, has one of the all-time best backs. Not a white man, of course... so a bit off topic.
As far as this thread goes... hmm, Jay did bring his back up alot over the years to rival Ronnie. I'd probably go with Jean-Pierre Fux as well.