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The most terrifying spell is Hypnosis. With Hypnosis, you can defeat armies several times stronger than yours, especially against neutral creatures; you can even defeat an army a hundred times stronger (for example, if you evaluate it based on the total HP). No other spell can compare to this, not Disintegration, not Hand of Death, and not Dragon's Breath. And if you also have the Martyr spell (Magic of Life, level 2), then Hypnosis + Martyr is pure bliss, or you can hypnotize the enemy hero.
Undoubtedly, the magic of Haven is the most powerful, even without Hypnosis. Just consider Mass Slow; how many times has it saved the day? Teleport, or teleport to a town (it was slightly adjusted in the expansion), and Forgetfulness, which turns almost any ranged attacker into a mere bystander...
As for Chaos, it's geared towards damage, but as someone once said: "The one who wins isn't the one who deals the most damage, but the one who takes the least."