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

Well, in principle, I'm not saying that you can rely on snipers 100%, but...
Still, you have to agree that the Rampart has an advantage over the Stronghold, first of all. The Barbarian army rarely appears in its full composition: there is a clear choice - either Cyclops or Ogres. Building both without a significant time gap is a feat in itself, and it happens relatively rarely.
The Rampart will only have similar problems with Dragons. So what? But the first 6 levels will be in full force, mostly outperforming the first levels of the Stronghold. Just close down the Snipers, and the Barbarians will have big problems.
And Blindness is a completely different matter... after all, it's magic, and the situation can be completely different. You can counter Haste with Slow, and Blindness with Heal, etc. It just depends on luck.
That's why I personally considered the option without magic.