I always use earth and air magic – they are optimal, powerful, and cheap. In my opinion, fire and water don't have strong combat spells, and they lack useful abilities like flight or city gates (although, if I'm not mistaken, there's a spell to summon a ship and sink it – sometimes that's useful :).
As for the optimal skill allocation, there seems to be a dedicated thread on that.
Also, all types of magic can be obtained in the Conflux, so I think those who have played it at least once can determine which magic they choose. I, for example, choose the types I mentioned above.
Question: Has anyone bought or acquired all the magic skills? And what is the effect of doing so?