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

a very difficult question: the sniper Jhelu, as well as the Dragon Wizards, may turn out to be useless if we have at least a well-developed Orrin with a bunch of archers.

I, to put it mildly, doubt that crossbowmen under Orrin will be better than snipers. Well, wizards might be better, but snipers? Unless you don't have enough money for snipers, then the logic is clear.

Krag??? Tazik is much better. At least 10 times better.

Well, Tazik is good for his own faction, the Swamp faction. Putting him in a Rampart should also make him fight well. But the Citadel is an attacking castle. I wouldn't risk, playing as the Citadel, exchanging Krag for Tazik.