Hearthpwn

Witchwood, Boomsday and Rastakhan.  NOT the other 3 sets people are mentioning.

If you don't have tons of dust, a good idea is to quickly disenchant all valuable cards from those three sets you don't intend on playing anymore (I chose one deck I will do quests with until April 7, and disenchanted the rest), and then craft a golden Leeroy and Velen (And maybe the other cards rotating out). Then in a hour or so you should get all the dust back + the option to disenchant both of them for even more dust for some crazy profit, if not playing wild.