The most useful spell.

I really like the Plague spell; if you take the Pain Lord, it helps a lot until you learn more complex spells.
Phantom is very useful against neutral creatures if you have a couple of strong stacks of archers or units that deal damage without taking damage in return.
Divine Vengeance is simply overpowered. Under the right conditions, the damage from this spell is just insane.
I like Berserk more than Subdue, I don't know why.
And even though no one mentioned it, Heavenly Shield shouldn't be underestimated either...