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These are objectively the best, which is why they are so popular.
Necropolis – I play with Tant (Galtan is considered the best)
Swamp – Tazar (Bronze)
Tower – Kira
Castle – Adel
Rampart – Kirr (Mephala, Roland, Ivor)
Stronghold – Dessa
Dungeon – Gunnar Jeddit
Inferno – there are no good heroes, so the choice depends on the situation. Paradoxically, I choose Pir.
In general, the best are logisticians and diplomats; they, along with specialists in destructive level 4 magic, are considered "cheesy" heroes, which proves that they are the best.