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In terms of strength, my favorite hero is the mage, as I believe that the magic of Haven is the most powerful in the fourth game due to its vast tactical and strategic possibilities: slowing, hypnosis, accuracy, teleportation (and in the fourth game, you can even teleport enemies around the battlefield!), dispel, and a portal to the town – all of this is Haven magic.
As for appearance... it's hard to say. The hero models in the fourth game don't quite appeal to me. Perhaps the best ones are the priest, the lord, and the necromancer.