Witch Doctor for me, currently at 54 and just finished Act 1 on hell. Really enjoying the class atm. Great AoE damage and survivability.
Been playing in a group with a Monk, DH and a Wizard with this spec:
http://eu.battle.net/d3/en/calculator/witch-doctor#acZXdU!ZbV!aaabZZ
Dire Bats have awesome range and damage and the Poison Dart with splinter rune along with attack speed on gear is great single target damage too.
13 sec CD on Soul Harvest (with a heal) and Spirit Walk (dodge an attack + 30% mana) are just crazy. Grasp and Horrify work great against some of the tough elites in Hell mode too.