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I believe a hero should be versatile. I don't divide them into classes; instead, I develop them as either a battlemage or a spellcaster. Logistics is promising, but on larger maps, Wisdom (the ability to cast spells up to level 5), Attack (30% more damage), Defense (-15% damage), Resistance (removes an upgrade from 3 units), Earth (most importantly, slowing, and then regeneration, explosion, and portal), and Air (most importantly, speed; the rest is just mana expenditure) are all essential! Ranged attack is also important if your army has actual ranged units (whose damage is comparable to, say, 30 Death Knights) or if you have cities like Stronghold, Swamp, Castle, or Conflux. Pathfinding is also useful (you can't prevent them from having to travel far... on larger maps).
Now, let's highlight the least useful skills (in descending order):
1. Eagle Eye. Undisputed leader. Useful on any map.
2. Scouting. You can roughly estimate the number of enemies anyway, plus or minus half. There's also the "Vision" spell. A few thieves won't make a huge difference to an army with a 20th-level hero. Useful on any map.
3. Tactics. In a battle against the AI, it will act on its own; you won't have to drag Dendroids across the entire battlefield. In a battle against a human player, cast "Haste," and the ranged units will attack someone on the first turn.
4. Ballistics. But there's Earthquake, Haste, Teleport, Air Shield, and more!
5. Navigation. Okay... I think maps with water just lengthen the game and take up unnecessary skill and artifact slots.
6. Estates. Useful if the map has few gold mines.
7. Leadership and Luck. You can use artifacts instead. Although they are good on medium-sized maps. Especially Leadership – you won't have to worry about a unit "deserting" at a critical moment.
8. Diplomacy. As someone correctly pointed out, it's a "cheater" skill.
9. Intelligence. A decent skill if the map is smaller than large, and you need to cast spells like "Regeneration" or "Portal" frequently.