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It all depends on the size of the map and the opponent.
Inferno: On a small map, choose Daleb; on a large map, choose Nimues.
Dungeon: On a small map, choose Leto or Sinithar; on a large map, choose Leto. Erin is generally not a good choice.
Sylvan: On a small map, choose Ossir; on a large map, choose Wingael or Anwen.
Stronghold: On a small map, choose Hagge or Tilshek; on a large map, any except Argath.
Haven: On a small map, choose Dugald; on a large map, choose Axel or Mive.
Underworld: On a small map, choose Inga or Karli; on a large map, any dwarf except Swei with a mentor, plus Inga.
Academy: On a small map, choose the one who uses fireballs / Hafiz; on a large map, it depends on the opponent.
Necropolis: On a small map, it doesn't really matter who you choose; on a large map, choose Lucretia.
Daleb was nerfed in the latest expansion, so I don't choose her very often. If you need speed, Grok will do.
Leto is useless against necromancers. But at high levels, witches give a good damage bonus for Erin.
Ogre is always a good choice for mages, because the hero's spells play a bigger role than his army. Many of the units there are not very good, except with artifacts. Otherwise, I agree.