This is not what is the most powerful class thread it is which class can become the most powerful.
Warlock - Warlocks are on the quest to become more powerful they always want more I believe it is this drive to become the strongest that makes them the most powerful they will give up mostly anything to achieve it ex Kil'jaeden, Archimonde, and Gul'dan some of the strongest beings ever encountered are warlocks. Need I say more