Even though it took Blizzard soooo long to put out WOTLK, I absolutely love their zone designs for this expansion.
I am allowing two votes since I couldn't pick just one. So feel free to pick two favorites, if you are so inclined.
I think The Storm Peaks is just plain impressive. You feel so small and insignificant there, as well you should by so many things related to the Titans.
And Howling Fjord is also really neat. It has a very Nordic / Viking feel to it. So those are MY votes.