Straight up, the only correct answer is Li Chen, the so-called Immortal Invincible himself. People keep bringing up side characters like Elder Bai or the Blood Demon, but they all serve as stepping stones or narrative devices to showcase Li Chen's absolutely broken cultivation system. The whole novel's premise is in the title, which isn't subtle. He starts with a cheat-level innate constitution and just snowballs from there, absorbing the power of every enemy he defeats. By the final arc, he's essentially a fundamental force of the universe, rewriting cosmic laws on a whim. The author wrote themselves into a corner with his power scaling, to be honest.
You see debates about 'peak form' versus 'potential,' but it's pointless. Even when he's temporarily suppressed by a higher-realm artifact, it's just a setup for another power-up. The antagonist in the last volume, the Void Devourer, was presented as an existential threat, but Li Chen defeated it by comprehending a concept beyond the established cultivation ranks. So yeah, it's him. Anyone else is just fighting for second place, which the story doesn't even really care about.