I'm not convinced that Mage is the best Arena class anymore. I think Paladin is much more consistent and Warlock has higher potential.
Paladin cards are just really strong. Paladins can have trouble with late-game minions, but Mages who don't draft Polymorph have the same issue, and Paladins have an incredible early and mid game that gives them board control to deal with late threats.
Warlocks have Imp Gang Boss and some huge tempo cards like Power Overwhelming and Voidcaller. Demonwrath gives them another (admittedly rare) AOE, and Darkbomb is Frostbolt-tier removal. Antique Healbot negates the class' main disadvantage, and as Arena has slowed down from what it was right after GvG their hero power becomes more and more valuable (especially since dilution by new cards means that the other classes except Paladin have a harder time drafting card draw).
I seem to be doing best with those two lately, at least, whereas I have about the same success with Mage that I do with Shaman and Priest.