It really depends on what you mean by powerful. The one poster who gave a summation of every school said it excellently.
If you mean most damage, you want storm. Nothing else does as high damage as storm, and generally, a storm wizard one-hits their enemies most of the time. But they fizzle, a LOT. Be prepared to fizzle roughly one in three attacks.
If you want to survive for a long time, you probably want ice or life. Ice has amazing health and life can heal itself better than any other school. Additionally, life rarely fizzles. Death is somewhat powerful for the fact that it can self-sustain, it does a fairly nice amount of damage and simultaneously heals the user for half.
Fire is the "in-between" school, with medium health and medium damage. If being a well rounded character is your thing, go for it.
Myth is a little bit unique, they get the best minions, minion boosting spells, and a variety of minions. It's probably the most different play style from a lot of the others. It can be somewhat powerful if you use it wisely and learn how to use minions, but it's not powerful in the same way some of the other schools are.
Balance is not my favorite choice for 'powerful', but they can be a great asset if in a party. They have strong shielding, blades, traps, etcetera.