The portal to the city definitely rules. Besides that, flight is great.
And during battles, chain lightning and ice lightning are very useful (with proper development of the corresponding type of magic). Also, healing can often be very helpful.
As for fire magic... the only thing that brought me real benefit was bloodlust.
At the beginning of the game (when there are still few units in the army and neutral squads), things like haste, bloodlust, and blessing are very effective.