I allowed myself to include 2 zones under the "classic" due to the vast quantity of zones it had to offer.
Classic (Pre Cata): Un'goro Crater & Felwood
Classic (Post Cata): Azshara & Silverpine Forest
TBC: Blade's Edge Mountains
WoTLK: Grizzly Hills
Cataclysm: Uldum
MoP: Hard choice... probably Dreadwastes or Townlong Steppes.