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I believe that the strongest are Knights and Wizards (excuse my French – I grew up playing Heroes 2).
A Wizard can significantly strengthen their army with mini-artifacts, and some units can be considered one level higher than they actually are. And Titans and Archmages can give a good fight – they can take out half the enemy before they even reach them.
As for Knights, they are simply powerful on their own. Plus, Griffons with their aerial attacks are a major headache for enemy archers. And on a large map, a Knight can turn their peasants into such a force over time that they can sweep away any Necromancer with his skeletons.