Only fire and berserk can save an impossible battle (when you're facing 7 stacks of 500 units each), because it affects 5 stacks at once, and they start attacking each other.
Earth and air are more useful on the global map... or for starting heroes with Meteor Shower and Tea Kettle, because of the strong imbalance.
My favorites are water and fire. The most essential, especially in campaigns (where strategy, not tactics, matters), are earth and air.