Fact: Storm is far and away the best school because Storm deals the most damage (in the right hands) and has the absolute best spell in the game - Tempest. Fire is the second best because it does second best damage and is well rounded. Also has the best single attack spell in the game - Efreet.
Others may have different opinions, but they are wrong. (Note: The first word and the last sentence of this post contain heavy sarcasm. Maybe...)
I would probably think Life/Death. Eventually, with the Death mastery amulet, Life could dish out Death spells just as fast as Life spells. A life wizard could have a life minion at their side spamming it's own heals, while a life could, as an option, use their pips to summon a death minion instead.
Conversely, I would say a Death/Life wizard would be just as powerful, with a Life Mastery amulet.
I would normally say Myth is the best, but, with the way KI works, Myth is never able to play at their full potential.
Storm is definantly the best school. Your spells are powerful, although you will have very few healing spells (which can be easily fixed by doing life as your second school. Storm also has very lousy aim, but I think that it's all worth it. 8)
My opinion, in order (I have one of each school/two accounts).
Death Fire Storm Life Ice Balance Myth
Death: Hard-hitting school with lots of high-damage spells. The ability to steal health from others to heal yourself is only an added bonus. Also, it has to be- otherwise, the Malistaire/Sylvia Drake subplot would not hold up; of course, my necromancers both hated Nightside, for obvious reasons.
Fire: High damage, lots of DoT spells and Power Link; I've haven't yet been on the receiving end of an Efreet, and never, ever want to be.
Storm: High damage, lots of AOE spells. Most Storm wizards I know don't even need traps or blades to do well over 2000 damage; even if their aim is terrible, they can bring the pain for very few pips.
Life: They have a ton of big attacks (ie: centaur, forest lord) and can use power pips on heals. Enough said.
Ice: Lots of AOEs, high health, and the tower shield that every other school dislikes.
Balance: Ability to cast spells that double as another school's (ie: spectral blast, hydra) and use power pips on Balance heals. Tons of great AOEs, though they don't do any real damage without critical, even with buffs, etc. Highly-elemental class, doesn't have enough of the Spirit schools in it- all of which get a boost on Balance, even though it doesn't get one back. Also the only school, apart from Myth, that has more than one minion- the best one of which is the little blue Mander. He traps, blades, shields, and occasionally attacks, but has very low health and can easily be K.O.ed in one round.
Myth: Lots of minions, hard-hitting (though not high-damage) spells- that earthquake is a Balance wizard's worst nightmare. All of those shields and blades you just spent the last 8 turns putting up? Forget it.
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Anyhow, that's my humble opinion. The haters can take it, or leave it.