Basically anything that destroys their board. Priest, Warlock and Warrior to some extent should do fine. Paladin has little-to-any comeback tools; Libram of Justice is not even ran in some lists. Answer their board and they are left with a few useless buffs in their hand.
I played against a few Paladins the past few days and lost against many of them.
But when I tried it out yesterday myself, I had a terrible time with it. Unlike my opponents, I wasn't able to perfectly curve out every single game beyond turn 5. I oftentimes ended up with too many spells and an empty board, or an empty hand and a lousy draw. From what it seems, a lot of people are currently teching hard against Paladin as well, and at least in parts, it seems to work.
No doubt a strong midrange deck, and Paladin might still end up as the best class overall when decklists are optimised, but the versions that I've seen certainly seem beatable. If we don't get a balancing patch next week anyway, the deck's winrate should drop a bit soon.