Tanatas
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In my opinion, the most useful resistance is Chaos. Let's assume the map is large. While we're busy leveling up, we'll encounter enough neutral opponents using Chaos + the Strength vs. Magic skill, which puts any opponent who wants to use Chaos + Fire Wrath in a difficult position. With Darkness and Light, it's significantly harder; their resistance doesn't have much of an effect – doubled mana cost, knockback, and reduced spell power are weak effects for these schools; essentially, it significantly hinders resurrection, vampirism, and both types of word spells. Summoning resistance is neither here nor there, especially if the opponent is a mage; doubling the mana cost is even laughable, but knockback and reduced spell power will help + in summoning, you can take Misty Veil and Return to Void (this is especially useful if the opponent is from Inferno). So, if I were to take resistance, I would only choose Chaos, and I would fill the other slots with more useful skills.