For me, in what I doubt will be among the most popular answers but who knows, its Cata's Well of Eternity.
I really enjoyed the three dungeons launched in the lead up to Dragon Soul for the story arc they presented moreso than anything else.
Well of Eternity in particular had just the coolest moment of both seeing Azshara in her full glory and looking over the ledge to see the Dragon Soul and the Well. It was just so cool to me that we stepped into one of the most important moments in the history of Azeroth and skirted around some of the most powerful characters to ever walk upon it.
