Hard to say. I guess the favourite from a perspective of the overall presentation is the Aszhara short. Well put together, tells an interesting story, looks good, with good voice acting.
And has some food for thought about the future stories.
From the book Tome of Blighted Implements [The Scepter of Sargeras, Part Four]:
Soon after the War of the Ancients, Sargeras had a vision : the Well of Eternity imploded once again, but this time, it dragged him down [2:59] to the very core of Azeroth. He was there for only an instant, but in that moment, he saw Azeroth’s slumbering world-soul–and in that moment, the world-soul opened one eye and gazed at the Dark Titan. He was enraptured [3:57].
So, not only it recaps the pivotal moment in the story of Azshara, but maybe hints that there is more to learn about the real reason of the fall of Kaldorei Emprire (will we get another time travel, I wonder).