What kind of spell do you like?

Combat spells are always useful for dark elves, as they have Arcane Wave, they can see the elements, and they may have the "Lucky Sorcerer" skill. But they are useless for other races if you play with large armies. With large armies, spells of dark and light magic play a very important role (but some of the strongest dark magic spells do not affect necromancers). Summoning magic can also help a little, but not as much as dark and light magic.