Which school of magic do you prefer and why?

That magic is great, especially the one whose book you managed to find! :))
Seriously though, without hesitation, I choose Earth and Air. I only take Fire if there's a Berserker available or a real chance of getting one (in the case of Ifrits, Black Dragons, or Conflux - Armageddon).
I haven't had much luck with Water, although the school is quite interesting.
Simply put, if you have Logistics, Attack, Earth, Air, and Wisdom, there are only 3 slots left. And you might also need Pathfinding, Defense, Intelligence, Tactics, Luck, and Morale... So Water gets left out.