I chose something useful for each faction. I just couldn't find anything worthwhile for the Elves...(((

For the Elves, the Enchanted Arrow is good; it's both a spell and an arrow. You can also use Deadly Shot, but only against a unit listed as a sworn enemy, for example, take a Lava Dragon with 280 HP, and there's a guarantee it will be destroyed.
You can also get extra arrows if there are sworn enemies.
For the Wizards, Magic Mirror and Wizard's Sign are good. Artifact Absorption helps when you have few troops left.
For the Demons,
Soul Eater is good when you run out of mana, and it's also good that creatures are removed from the battlefield so they can't be resurrected or raised as undead.
Hellfire is good in itself, it's a shame it consumes mana, but it can be
restored with Soul Eater.
For the Knights
Prayer, because the Knight's strength lies in his army.
Surveillance is good because the hero escapes and gets to attack again for free.
Warlord, if you focus on training, which is especially relevant because after patch 2.1, you have to retrain high-level creatures.
For the Necromancer
Eternal Slavery
The Necromancer's strength lies in numbers, so this skill is needed.
Spiritual Link is needed because the Necromancer doesn't have a huge mana reserve.
Banshee's Wail can be used so that the opponent doesn't get many turns with a high morale stat.
For the Dark Knight
Arcane Waves, to destroy the enemy faster.
Sense Elements, a small bonus to damage from spells and creatures.
Dark Ritual, I rarely use it, it's a shame to waste a turn restoring mana, but when you're waiting for a siege on your castle and you have no mana, it helps.
For the Dwarves
Completed Rune, naturally, hunt so that the rune is cheap.
Reforged can be used because a lot of resources are spent on it later