The strongest hero.

To help you objectively assess the characters, here is a list of each one's accomplishments.
GODRICK. Participated in victorious battles against Markel and the Overlord.
NICHOLAS. King and bearer of the Griffin Heart; hardly a weakling.
TIERU. Perhaps the most powerful mage in Ashan, creator of the Griffin Heart and the Ritual of True Nature.
AGRAIL-RAILAG. Killed Nicholas, Wier, and Trals, managed to defeat all the Igg-Shail Clans, and then participated in the battle against the Overlord.
OVERLORD. The most powerful demon. Probably...
BIARA. Her strength lies in cunning and the ability to weave intrigues. That's why she killed all her enemies with other people's hands.
MARKEL. Plundered the Silver Cities and killed Cyrus.
ARANTIR. Killed Giovanni and Orlando; also managed to seize the Shadow Skull, which he lost in the battle against the Dark Messiah, in which he also perished.
ILAYYA. Managed to unravel Trals' plans and persuade the Black Dakons to return. Participated in the battles at Thor Hall and Claw.
FAIDAEN. Defeated Biara twice and killed the High Vampire Nicholas. Participated in battles against Markel and the Overlord.
ZEHIR. Liberated the Silver Cities, participated in battles against Markel, the Overlord, and Biara.
GOTAI. Killed Alaric.
FRIDA. It was her spell that destroyed Laslo. She also participated in the battles at Thor Hall and Claw.
Zehir, by the way, also killed Rolf, Wulf's brother