[Shovah32]

As said, placing is very important. Here are a few good spells from each path:

Fire:
Fire Darts can make a high (f9-10) fire pretender into a nice early game artillery mage (number of shots scales with level)
Fireball, its good range, damage, has AoE(1 square but still) and ap.
Falling Fires is nice, decent range, nice damage and a fairly big AoE
Pillar of Fire is a monster, huge range, massive (AoE 1) damage and can set alot of units on fire

Water:
Falling Frost has good AoE, nice damage and can freeze, need i say more?
Frozen heart has great range and precision and does some very nice damage

Air:
Orb lightning does lots of damage and has alot of shots, its very close ranged though
Lightning bolt has good damage, range and precision. good all round spell
Thunder Strike has nice range, AoE and great damage. Quite high fatigue though.

Earth:
Bladewind, the king of EA. Any mage with some sort of precision (a bladewinder who can cast eagle eyes is godly) can decimate almost any EA army with this spell. Try a marveni communion with this (4-8 slaves, 2-3 druid masters with earthpower and eagle eyes can rain bladed death)
Petrify, the holy grail of SC destruction. If the target fails a mr test they are gone and thats that, even if they pass they get paralyzed so you get another shot.

Astral:
Mind burn, great range, precision and nice damage. MR negateable though.
Soul slay, an instant death spell if they fail a mr test, whats not to like.
Paralyze is like soul slays little brother. it wont kill the enemy on its own but can give you plenty of time to finish them
Nether Darts is another beast, it has alot of shots all with good AN damage and a chance to feeblemind the enemy

Death:
Raise skeletons, this spell is another very powerful (and over used) spell. Fairly easy to cast, a couple of death mages can raise an army in a few turns and, with a few relief casters can keep it up for the whole battle
Plague, nothing says i love you like a single spell dooming the enemy army to a terrible plague.
Cloud of death, does nice damage, has AoE and lasts for a few turns
Desintigrate, read soul slay
Dust to dust, the ultimate (sort of) anti undead spell

Blood:
Agony is great for routing conventional armies
Bloodletting, a few castings of this can decimate entire armies, with each casting hitting the whole battlefield
Life for a life, 1 slave lets you hit a single target for 40+AN unresistable damage

These are all just battle spells, not including any buffs or spells requiring gems (although the blood spells need slaves) and is nowhere near a list of all good spells, just those that came to my mind.