There are hard in so many ways.
Decks that have very smart plays that req. alot of think seems to be the hardest for me, Priest come in my mind as winner because of though steal, mind vision, circle of healing, pyromancer, cleric.. and so on.
Arena Warrior is hard to win with, because it req. alot to make a solid deck balanced for warrior: Minions/Wepeons/Health, Early & Late.
Warlock whenever to tap or not tap and calculating risk vs reward. Math is hard. (Math: Outs/Risk)