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

Not really "that good." In the early game, up to levels 5-7, it can be useful. But even then, it's not great. Later on, Fire Magic won't be very helpful in battles. Considering that Adrian's specialization implies "Expert in Fire Magic" from the start and doesn't provide any other bonuses, it's more beneficial for long-term gameplay to choose any other sorceress and develop her Fire skills if it's really needed, although it's better to focus on Earth and Air or Water. :)

And regarding rushing... After a couple of Inferno spells, the hero runs out of mana, and instead of rushing, they start searching for mana wells. :)