This isn't one I have a pat answer for, because there are a lot of beautiful zones in WoW. There's Netherstorm, which has an alien, sere beauty to it. There's Howling Fjord, and Grizzly Hills, and the Dragonblight... heck, Wrath of the Lich King has a whole host of beautiful zones in it. There's Deepholm, which combined the alien vistas of Outland with a vaulted enormity that I've always liked, and Uldum, which looks spectacular even if I'm not as enamored of the questing experience. And Mists of Pandaria had the Jade Forest and the Dread Wastes, two zones I was simply blown away by when I played the beta and which I've liked ever since.
So since I can't answer my own question - I can't pick a single zone and say this is the most beautiful zone in the game - I guess I'll ask you, instead. Is there a single zone that you think of as the most beautiful? If so, which one?