I would say Dragon Soul. Not because of the emptiness between Dragon Soul and Mists. Because Deathwing really didn't have an exciting battle, like for example The Lich king. I remember having fun when attempting Lich king, getting the strat right, etc. It was like, "omg finally!" after our first kill. Even after killing Arthas several times, even letting people get carried off to die on purpose, my guild still had a blast. We didn't get that with Deathwing.