Secondary skills in Heroes 3.

I try to give all commanders wisdom, logistics, earth magic, and ballistics. Luck is also useful (if there are no corresponding artifacts). The rest depends on the class. For warrior commanders, tactics, leadership, attack, and resistance are useful (especially for Inferno with its powerful but not very durable monsters, as well as for the Conflux); sometimes defense, ranged attack, water magic (for the Castle and Conflux), or air magic (for the Tower). For wizard commanders, magic, water magic, and air magic are useful; sometimes tactics. Fire magic is useful in the Dungeon and Inferno. A city defender commander benefits from knowing artillery. Mysticism is useless; vigilance, education, literacy, intelligence, property, and first aid (the tent is quickly destroyed) are of little use, as are scouting, navigation, and perhaps pathfinding.
Also, I don't quite understand diplomacy: at first, there won't be enough money to buy local monsters at an increased price, and then there won't be any serious local monsters whose recruitment would bring more benefit than harm (due to the drop in morale).