depends on the spec, and depends on the encounter. In arena support, druids, priests and mages are troublesome. Druids are capable of locking a healer down completely while keeping their people healed with HoTs. Priests can mana burn and mass dispel your shield. Mages have a painful counterspell and their burst magic damage goes through BoP.
Solo, for a healer rogues can be bad and any class that can kite like hunters. I have a hard time with a lot of classes 1v1. Warrior is one of the easier ones. Long fight, but i usually win if given the time. Spam concecrate, spam FoL with JoW+SoW up.
Prot rips through melee.
Not sure how ret does anymore.