Control Priest

The Idea with your mulligan here is to find most of your early game survival cards as well as a decent card draw engine, Northshire Cleric. Playing Northshire turn 1 will have most experienced players thinking more critically about throwing out early game minions, and even if they do, trading in to a 2 or less attack minion  and hero power-ing your Cleric is a better way to spend some of your early games than just healing face and passing. Look for the more critical minions for your Shadow Word: Pain and Shadow Word: Death to eliminate, and generally be stingy with your cards. Bone Wraith is probably the least priority card, since you have to wait until turn 4 to play him, but against certain aggro decks like Murloc Paladin and Combo Priest, he can be a life saver, letting you stay alive until you can play your stronger taunts with deathrattles. And finally, playing Doomsayer on a board where the opposing player is unable to clear him is a great early game stall.