The most useful spell.

Massive Dispel. The entire enemy army doesn't attack or retaliate. Combined with the "Sharp Mind" ability, it's an incredibly powerful combination. And with the "Sandro's Cloak" artifact, it becomes even more devastating. For example, against mages, you can immediately disable three stacks of ranged units: gremlins, mages, and titans. Of course, if you're playing against a Necromancer, Dispel is useless, unless you have Sandro's Cloak.