That's a tough one.

I started boxing in Burning Crusade, and that is my favorite era of Warcraft.
But my current favorite team is Death Knights, which weren't around back then.

I wouldn't mind if it was the BC world, with tough heroics and the raid content which I loved the most.
But with all the race/class combinations that are available up to and including Cataclysm.
Still a level 70 cap, and the old style of talents that did not force very cookie-cutter builds, with the exception of the final 10 points.