What is the best duel character combo in World of Warcraft?

None. Each class has counters it can use against the other class: warriors can use shield bash against spell casters, mages can use frost nova and blink, priests have a simple group fear, shaman have totems to slow others, druids have thorns, paladins have a stun, Rogues have stunlock, warlocks have individual and group fear. In the end it truly is the person and the armor/weapons NOT the class. In 2 v 2 some combos are pretty cool synergistically: Druids/Warlocks - warlocks have efficient DOT damage and the druids can do HOT heals on the fly. Priest/Warlock similar to druid warlock - by the end the combo can get ~7 damage for each mana the priest expends. Druid/Shaman - very good dps/outdoor abilities Mage/Druid - AOE and healing Priests/Paladins are very hard to kill - they both heal, and the Pally wears plate mail.