The strongest hero.

Actually, that's not necessarily true. Markel clearly stands out among other Necromancers because he used to be a Knight, not a mage. And his methods are special in this regard. While he has mastered necromantic rituals and easily controls the undead, unlike mage-necromancers, his main weapon is a sharp tongue and the mind of a true intriguer.

Biara, for example, was even better at intrigues. And if you judge a hero's strength from a plot perspective, then if she had truly had her own will, she would have destroyed everyone without resorting to weapons. And only by uniting could they defeat her.
Kha-Belekh is also cool, with his blitzkrieg. But the coolest of all will be Sareth, who, the scoundrel, killed Arantir...