Light or Dark magic.

Dark magic is more effective than light magic because it makes the enemy's troops fight as if they were on your side, which means you can win battles with minimal losses. It doesn't work against necromancers, though; in that case, the Sandro's Cloak artifact would be helpful, but it's not always available. Divine Vengeance is a strange magic; to make it useful, you need to lose half or more of your troops. I level up Dark magic for all heroes except the Dark Elf; for him, I think Arcane Wave is better. For the other heroes, I don't level up magic at all, except for Summoning occasionally. Instead, I prefer to level up Luck, Leadership, Attack, Logistics, Defense, or Mechanics – it depends on the situation.