Which Hero from Heroes of Might and Magic III is the best?

I think this isn't about Xeron from Inferno, but about Xiron the demon lords from the campaigns )) I don't remember his abilities... although maybe it's the same person)))

No, Xeron's specialization is Inferno (spell), while Xiron's is devils/archdevils.

I voted for Crag Hack, with Sandro in second place — in my opinion, they have the most useful specializations. Katerina and Roland get a "crowd favorite" award from me — their specialization is on my favorite unit in the game. Xiron, Kilgor, and Mutare are pleasing with their specialization on own higher units, but overall that's the only thing that makes them stand out. As for Jaelu and Dragon, I would choose Jaelu, because snipers might be a lower level than Wizards, but there are usually far more of them, and as a result, they are more useful.