Snow Princess :)

Free card - and a good one at that.  Plus you get candies to feed her quite the amount of xp.  Now instead of dual Anna you could go with dual Snow Princess for 4x ATK/RCR or one of each for 2x HP/RCR and 4x ATK.  The 7-star ATK cards don't have a lot of RCR, but some of the lesser star ATK cards do.