The strongest hero.

Period, period!!! Why is there a period at the end of the topic title?

It seems to me that, considering the rules, the topic title sounds incorrect. And besides, judging a hero's strength based on videos is questionable... The topic is more about the plot (who defeated more enemies) than about the actual strength of the hero.

But, setting aside these points, except for one: I also take character progression into account, and I will say the following: Zephyr seemed to be the strongest hero to me. He is the main hero of the game, he defeated the most enemies (not based on videos, but based on the plot), and his development as a mage is the coolest. Although, Biara did kill Isabel. But Railag defeated Wayer. And the Overlord was killed by everyone together. Sigh...
Perhaps, there is no definitive answer. The developers did not show any hero as being stronger or weaker. But still, I will answer "Zephyr." Based on personal preference :D.