#26 - June 10, 2013, 3:35 p.m.
For me it was 3.0 (or Wrath of the Lich King launch, more accurately). Northrend is just a piece of content that I still find captivating (particularly Howling Fjord, Grizzly Hills, Storm Peaks and Icecrown).
If I had to pick a particular patch, 3.1: Ulduar!