discussed ad-nausem in this (and many other) forums. (Can search for all the leveling thread around Legion launch date)
Level as Prot. Do WQ as Prot.
/endthread
Protection is brokenly good at leveling. It's not even close. Retribution is also good for world questing at max level with a bit of gear, but it still loses out to Prot when it comes to elites and areas where you can pull 5+ mobs consistently.