Mass slow is probably the best spell in the game, as well as teleportation to any of your cities of your choice, and suffocation, which are also good mass defensive spells.
Water magic is only liked because of mass blessing, a very powerful spell, perhaps second in usefulness after mass slow, but apart from it, there is nothing useful in water magic.
Air magic is also not bad, with many attacking spells, and useful abilities like mass haste, lightning, and chain lightning, but with earth magic and all its spells, the only useful thing remaining is probably chain lightning.
In my opinion, fire magic is the weakest magic, with no useful spells at all.
I only voted for earth magic because its advantage over the others is simply undeniable.