Many voted Grizzly Hills. I must agree that it's one of the best designed zones overall. The only thing I'd change would be the annoying opening music. Rest of it isn't too bad.
The other one would be Howling Fjord. I knowingly chose it as my starting zone, and it never stops to amaze me. The entrance is epic with Utgarde castle towering in the background. Boring tundra is so.. boring. The best part of it is the starting keep. Rest of it is just.. well, boring and ugly, the most interesting things being the crashed necropolis, the temple city and Coldarra.