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GGA Dizzy hit the nail on the head with mage and arena. Mage hero power is one of the best arena powers in the game, and mage basic cards flamestrike, fireball, polymorph, frostbolt are ridiculously powerful in arena and you're pretty much guaranteed to get some of them in your draft. Personally I hate drafting mage because I think the decks are uninteresting and boring to play, but if you're going for wins mage is one of the safest bets.
I've only gone 12 wins once with warlock. But it was cool because according to hearthstats I was among only a small percentage of players who were able to hit 12 with warlock at that time.