It's been by far my favourite expansion. Fantastic class design, an absolute ton of fun new features and modes introduced, far better storytelling than previous expansions, easily the most fleshed-out and polished new race they've added (both in terms of the player models and the lore), and, most importantly for me, a huge focus on things that are *new*, rather than dredging up some old villain and coasting by on nostalgia appeal.