My opinion:
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Warlock. They can use any form of chaotic magic, no matter how dark, and no matter what it costs. Warlocks can use necromancy, void, fel, etc.
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Mages, because the most powerful characters in the story are or have been mages, such as Medivh, Azsharah, Illidan, Jaina, Archimonde, etc.
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Warrior. The strongest warriors are directly empowered by the Titans and by Valor. It’s not a coincidence that Sargeras has specifically tried and failed to recruit the Aldrachi, a warrior tribe on a distant planet, for the Burning Legion. The only 2 instances, where Sargeras was wounded was by the Leader of the Aldrachi and by Broxigar. 2 Warriors. Varian Wrynn for example was able to fight through Guldans entire army alone. I think warriors are weaker on average than Death Knights and Demon Hunters, but they can become way stronger in terms of growth. Also, theoretically Sargeras is a warrior.
I think Demon Hunters are gatekept powerwise by the Demons they can consume/tolerate, and Death Knights are a constant in terms of power after they are raised. For example Illidan drank from Guldan’s skull, and afterwards he went to black temple and became a villain. Evokers call upon the power of the aspects, so they can’t just train or seek more power to become stronger, unless they change their class, such as this yellow dracthyr, who became a warlock.
I think in general “egoistic classes” are always gonna be stronger alone, but classes like Druids, Shamans, etc. in the lore are always the best in groups. Think of Anduins scene, where he mass-ressurected his entire army against the Horde.