I mean, ultimately this all quite subjective. And obviously, my headcanon of Yaka is NOT at all what he is actually like. And you are obviously right that one has to separate gameplay and story a bit, at the very least.
That why I wanted to know what you guys' ranking are. Thats the fun, after all.
Ok, I will make a list. But it will be some kind of all time ranking.
1. Merlin in his prime / All-Devourer
Going by achievements and capability, those two should be pretty high. Merlin managed to beat the traitor wizardkings (even if he got some help) while still learning all the magical spheres. Stuff like beating Yaka "God of fire" when being an apprentice fire wizardking is quite impressive. After beating the traitor wizardkings he got captured by the All-Devourer but despite being constantly tortured by it he managed to gather allies to fight the shadow demons and ultimatively managed to beat him in a final showdown with his allies.
All-Devourer is up here, because so far he seems to be more capable than Lithyl. He did menage to enslave the syrons for aeons and nearly conquered Athla. Seemingly by first seducing some wizardkings into using shadow demons what led to their doom and later on with some kind of invasion, but his machinations fell short in the end. Though, no true immortality for him it seems, which is a bit weird I think. He should have had godir powers at that point.
2. Sundren
Started as a little rebellious princess, uncovered the plot of Werlac with merlins help, created the torchbearers, won the big war (I think it was kinda vs elves and empire in the end) and managed to barter for peace after uncovering the evil plot.
Then when Werlac succeeded anyway she managed to fight of Lithyl and even captured her even if it took decades. Then was essentially manipulating the two major factions of Magehaven and managed to open the worldvault to aid in the eternal war against the shadow demons. If it was a good idea still is not yet clear. But she managed to break a prison that is older than Athla.
3. Lithyl - Eldritch Sovereign
As humanoid she clearly failed. She was one of the first to invade Athla (havent seen anything about leading anyone) but clearly got beat up by Sundren at some point. Got captured by her and corrupted into an Eldritch Sovereign then freed by Merlin, which she promptly corrupted. Is now in control of the Umbral demons and nearly managed to destroy Athla, similar to the All-Devourer. She has some scarry powers it seems but it is hard to gauge how common those are so far.
4. Werlac - Aow3
Not very powerfull on his own but managed to essentially control the whole of Athla from the shadows. Got stopped by the torchbearers but somehow found another ally to help him reinvite the wizardkings of old. From then on it only went downhill for him. Nobody gave him the "thank you" he hoped for, no wizardking gifted him powers or immortality and he resorted to allying up with Urrath, becoming an unimportant minion as jailor in some distant umbral corrupted realm.
5. Afred Elderstone
A young godir that managed to unite the humans of Athla against the Wizardkinginvasion to fight back. He helped repell Lithyl and shows great potential to become a key asset in fighting Urraths corruption.