This thread is twenty pages, but I still want to stick my post in here too.
Sentinel; The Pandaren Brewmaster--
-- Incredibly powerful, with one of the best passives in the game, highly underrated, and amazing survivability with the Primal Split, excellent early-game harasser if you're paired with someone that got a RoB, or if you get one yourself.
Undead; The Nerubian Assassin
-- Obviously commonly played, greatest early game harasser ever, in my opinion. Mana Burn and Impale let you keep doing damage with little fear of retaliation, and early game vendetta gives you a quick escape option. Also has one of the best passives in the game