Fireball: 4/5 the only good ranged magic attack. Available from the start and keeps you company till the end of the game.
Flaming Weapon: 6/5 its damage output combined with multi-hit maneuvers is far higher than damage other spells can produce. Overpowered.
Ignition: 0/5 useless, I never used it
Fireworm: 1/5, setup requires static enemies which sux, needs to be cast 3 times, crappy if you ask me.
Freezing Attack: 2/5, requires melee range, I used it once and then went back to Maneuvers.
Energy Drain: 0/5 useless, I never used it due to mana pots and Recovery song.
Magical Illness: 5/5 The only damage over time spell in the game, makes great use of Rush, can be cast behind walls/corners.
Frozen Blood: 5/5 Double the punishment, double the fun.
Armor: 5/5 makes your characters nearly immune to physical damage (tanking swamp soldiers and gryphons is where this really shines). @Queeg: it's better to cast Armor and play Genius or other useful song.
Web: 1/5 Useless at the start, costs too much mana. Useless at the end, you don't need it. 1 point for being one of the ways to defeat the Troll.
Wall: 3/5 If your defenses suck, you can use Wall as meat-shield.
Summonning: 1/5 useless. 1 point for being requied to open the city center.
Enemy Status: 3/5 Annoying to recast as it keeps dropping when out of combat. Useful info for new players, but not really required.
Storm: 2/5 I never used it, but it can be cast behind walls/corners, so a caster group could make use of it.
Invisibility: 3/5 great value during the village defense, but not really required anywhere else
Call In: 5/5 Required to finish the game + saves a lot of time when doing areas without fast travel (Dragon).
Magic Missile: 0/5 useless, I never used it.
Automapping: 3/5 agree with Queeg
True Seeing: 2/5 Gives tips when you get stuck, but it's not really required. Annoying to recast it over and over.
Control: 4/5 Turning your enemy into a meatshield is always useful. Short duration.