Hmmm... Remove the Bugs / Glitches, and NPC's / some events (Depends), so I I can focus more on either Story, Visual, and/or Gameplay enjoyment
Classic: Visually, Teldrassil. Gameplay, Alterac Valley.
Burning Crusade: Pretty split on Zangarmarsh and Eversong
Wrath: Howling Fjord. Hands down. On of my all-time favorite zones in the game.
Cataclysm. This is hard, because I largely didn't like the expansion, and the world changes. But unique to the expansion without old-world zones, I'd say Uldum and Gilneas.
Mists of Pandaria: Jade Forest, Kun-lai, and probably Townlong. They're very unique zones apart from the rest of the game.
Draenor: Frostfire.