Alchemy - excellent for races that don't get access to trade - I can't even imagine playing dark elves in WBC2 without Transmute, so much useless metal. Bronze golems are also very good summons for their mana cost. Create Item is OP in WBC2, kinda useless in 3. Good magic sphere overall.
Healing - always useful, but less so for elves who get Elcor anyway, minotaurs, or swarm-style races.
Illusion - underrated in my opinion, very strong debuffs that can disable entire armies. It requires your hero to be near enemies though, so it's probably better for hybrids than pure casters (or at least you need enougth speed to run away). Some races already have easy access to psychological effects so it might not be such a good combo for them.
Pyromancy - as you said, it's a great versatile magic type. I found the damaging spells to be too inconvenient to use most of the time, but the buffs are excellent. Soul Flame and Berserk are excellent for races that don't have access to them normally, Firebreath can turn even the weakest units into building destroyers (it's especially good with skeletons or husks for destroying towers). Fire elementals are not very tough, but they make great hit-and-run units and can cause fear.