The strongest hero.

I agree with many that the conditions for the answer are somewhat incorrect, but I will still express my opinion. Of all those listed, Kha-Beleth clearly stands out. He is one of the seven demon lords (if I understand the plot correctly), in any case, his status is clearly higher than all the others, and he is on par with Elrath, Arkat, and others. He has the largest army, approximately equal to all the others combined, if not more. All the others fear him (except for Railag). His only weakness is that he is locked in Sheogh. So, it is somewhat incorrect to put him on the same level as the others; he will always stand out. However, I still voted for Railag. Of those who are featured prominently in the campaigns, he is definitely the strongest (perhaps along with Arantir). As an example, consider the scene before the final battle with Kha-Beleth, when the Overlord offered Railag/Agrail to rejoin his side. At that time, everything depended on him. And even in the conversation with Kha-Beleth, he acted like the leader of the group. And Railag's ability in Lords of the Realm is the most devastating. From level 30, he is simply invincible, and even starting from level 10, it's not a walk in the park.

Actually, Agrail knelt before the power of the Overlord, Zehir is still a young one, Arantir was defeated by Sareth, and Sareth himself knelt before Kha-Beleth, and there's nothing to say about Faydaen...

I think that Agrail didn't kneel before anyone but pursued his own goals when "joining the service," and even in the guise of Railag, he didn't show too much respect to the Overlord. Zehir and Faydaen, I agree, they are not contenders. Well, being defeated by the Dark Messiah is nothing to be ashamed of, and uniting Erathia and becoming the second lord of all of Erathia after Belkhet is something not everyone can do, so I wouldn't dismiss Arantir so easily. There is no canonical information about Sareth; in 2 out of 4 endings, Sareth chooses the side of his mother.

Regarding the battle between the Overlord and the four. I think Kha-Beleth deliberately surrendered to allow the heroes to leave with the false Isabel. Otherwise, how can we explain that the huge demonic army did not participate in the battle with these four? If the Overlord wanted their death, I don't think they would have calmly left Sheogh. Moreover, Kha-Beleth achieved what he wanted - the Messiah will soon be born, and the Overlord's best agent, in the guise of the queen, rules the Empire.

Deliberately surrendered!? and disgraced himself in all of Sheogh? and released the rulers of the four main powers of Ashan and his mortal enemies from his lair?! The huge demonic army was defeated by an equally huge combined force. The best agent certainly caused a lot of trouble, but one of the results of his activities was the defeat of almost all demonic sects and cults in Ashan, so it is very unlikely that Kha-Beleth surrendered in the battle for the dubious opportunity to put Biara on the throne of the Empire for a short time