It’s vastly vague. Some characters gained immense power, and some character lost power.
The Strongest Mage is Without a Doubt Azshara, before the influence she was close to Archamond and Kil’Jaeden’s scale.
Broxigar had impressive feats and even carrying a more impressive artifact. In raw power I would have to say the strongest warrior could possibly be Odyn or perhaps Sargaras.
Arthas maybe the strongest Death Knight with the Helm of Damnation and Frostmourne, but he’s certainly not the most cunning.