A lot of streams I watch, and from my personal experience from last arena which was 8 wins. Mage, mage, mage, mage, paladin, mage, mage and paladin. I also lost against mage, paladin and a mage.
Aand I played a mage..
There has to be some balancing issues with the classes when in a random card pick scenario they just perform a lot better than other classes.