The most useful spell.

There is no single most useful spell! "Subdue" helps to kill legions of neutral creatures, but it is not very useful against an army with a hero. It is better not to risk casting single spells against a mage - you know what will happen ;). Sometimes "Berserk" is better than "Subdue", and sometimes it is better to hit the enemy with "Earth Shock". So, you need to think carefully. "Divine Retribution" is also very useful - the more enemies the target has killed, the more they will get in return.