This will surprise no one: Lor'themar Theron
Leader Story for him and got a better understanding of who he really was. But more than that, we got to see a fair bit of him in Mists on the Horde Side (and even a little on Alliance side).
He played a pivotal role in the Dominance Offensive Dailies in 5.1 and again on the Isle of Thunder and it's given me a much better feel for his character, enough that I can't help but love him.
He's a man thrust into a role of leadership he never asked for or wanted and left with no real choice but to do his best to protect a people beset on all sides and already decimated by war. He will do whatever it takes to preserve what's left of his people regardless of what else he might have to betray. And he's still less crazy than Jaina.
I'm glad he finally got some lore attention and had it done right so that I think people (like me) who'd been indifferent to him finally got a chance to see his character in action. For the Glory of Quel'thalas.
Vol'jin's still my second favorite and was my first pick for Warchief so I'm ridiculously thrilled that's happened and can't wait to see how it will play out. I think he'll be a great Warchief, though it will be interesting to see if the fact that he's not an orc will end up causing friction down the line.
Postscript: In looking over my post for yesterday I couldn't help noticing that Genn's shorter than Tyl. All the other leader models are larger than life (and player characters) but Genn seems to just be ordinary human sized. I can't help wondering why that is.