The strongest hero.

According to the plot, he is clearly the Overlord.

If I understand correctly, this is about the game's plot? But here, the obvious favorite seems to be the developers' pet – Zehir, who is young, promising, powerful, and hasn't lost a single battle.

commanding the army (meaning defense, attack, and all bonuses for the troops)

Please, elaborate. If you mean the art of command: two roughly equal armies are given to the heroes, and whoever wins is stronger. I don't quite understand, especially about the bonuses for the troops.

Gotai (after capturing Shahbadia, he receives large bonuses)

Although I don't quite understand the meaning of this statement, I will say that Gotai should not even be on the list of the strongest.

Whoever has the stronger magic – it's clearly either the Overlord or Markel.

Markel, yes, is a strong mage, but the Overlord hasn't been known for creating great spells; on the contrary, his magical abilities were not highly regarded. And here, the candidacy of Railag should not be overlooked – he has not only Warlock magic but also Tieru magic and the magic of the Order of the Dragon. And if there is Markel, why not Arantir?

One-on-one – the Overlord will definitely win.

I talked about the conditions of the battle earlier. If you tie Kha-Beleh's hands and feet with some super-powerful spell, immobilize him, and equip his opponent with powerful artifacts like the Skull of Shadows, who will win? Of course, this situation is very unlikely, but the conditions of the battle play a very important role.

He is the master of the fate of Igg-Shail and (partially) Isabel, but not of all Ashan.

So, the head of one of the 7 powers of Ashan is not the master of its fate? Hmm, in that case, only Sareth can be considered one of the masters of Ashan's fate.

He could not eradicate the demon worshipers; Igg-Shail united them for a short time.

It seems that after the final cutscene, it is clear that Igg-Shail is united, and the demon worshipers have been completely eradicated.

His participation in the campaign against Shi'o did not really give anything to Ashan.

Well, yes, except that they seriously damaged the demon army and showed all of Ashan that it is possible to safely go to Shi'o, give the Overlord a good beating there, and return home safe and sound.

The fact that he is the heir of Tieru is, of course, commendable, but it does not play a decisive role in the fate of Ashan.

He is not only the heir of Tieru but also the only (or one of the few) heir of one of the strongest and most glorious orders in Ashan, with a thousand-year history, knowledge unavailable to others, and other things that come with it. The Order of the Dragon has always been a serious force in Ashan, and all its heritage has passed to Railag.

The Messiah is born, Ashan is in ruins.

The Messiah is born, but it is unknown which side he will take (and it seems to me that Ubisoft will prefer a happy ending). Ashan is in ruins, but the demons have also suffered very heavy losses. And I saw an interview last summer where it was hinted that a duel between Railag and Sareth is quite possible, so Railag will still try to stop the Dark Messiah.

For some reason, it seemed to me that Tieru was not the only Knight of the Order of the Dragon at that time (maybe I'm wrong).

It is quite possible, but I heard or read somewhere that Tieru is the last. Of course, I may be wrong.