I personally like Earth and Air magic.
1.Earth is powerful (mass slow -> a couple of explosions and half the army is gone)
2.Air.... I just really like Chain Lightning and Fly...
P.S. Water and Fire are cool, of course, but waiting until you get Armageddon or Prayer... takes at least a long time...

Yeah, I like Earth and Air too, but I'm doing fine without them as well.
P.S. As for Explosion, I think it's unlikely you'll get it faster than Armageddon, but Armageddon is unlikely to be useful in battle either; now Prayer is magic worth having!! AND a teleportation door will be more useful than Fly (with it, around the world in 2 days!)!!!
And what do you think of Restore??? Or Remove Spell??? Slow and explosions are only good against neutrals; when fighting a human, everything is much more complicated!!!! :)