Tauren is probably my favorite, because Mulgore and Thunder Bluff are both gorgeous from the air. I'd be really interested to be able to fly around Silvermoon, though.
I am extremely annoyed at Orgrimmar, which is by far the ugliest city in the game. Stormwind is extremely well done in comparison. I like certain types of Pandaren architecture, especially in Jade Forest, but the Shrines are so completely uninspired. It's obnoxious that the two most common places for Horde to hangout are two of the worst looking areas in the game. It's why I use the auction house in Thunder Bluff and AFK flying above Stormwind.