Tirisfal Glades. I enjoy the story of the Forsaken pulling themselves together after waking up from being the Scourge, working who's in their grave and who isn't, who is still with the Scourge and who's broken free. You encounter several spellcasters on your journey through Tirisfal Glades, Undercity and Silverpine Forest, all of whom knew each other in the Kirin Tor, and they really pull all that together for me. The zone has a somewhat post-apocalyptic feel as the Forsaken scavenge what material and people they can from the ruins of Lordaeron.
The Chaos starting zone in Warhammer Online is a strong contender though. I didn't get the most out of it as my system couldn't handle it. But I remember the moment my Marauder stood just outside the portal, looking down at the Chaos staging area and the Chosen waiting for him, and I looked at my toolbar, and clicked on a button, and my Marauder's arm turned into a monstrous mutated claw.
A monstrous mutated claw.
It was a happy moment.