My run down:

Abjuration: A must have. Dispel Magic. Endure Elements. Protection from Arrows. Lots of things that keep you alive or stop magic. Not having abjuration is like not wearing pants.

Conjuration: teleport. It has teleport. Yeah it can make webs and clouds and monsters but teleport. That's all there is to it. The rest is a bonus.

Divination: You can't ban it!

Enchantment: This is one of those things you need or one of those things you don't. It depends on the game. Maybe your wizard can get through life just by killing or threatening to kill anyone in their way. But if they're part of society instead of loners in a tower or adventurers in the middle of a dungeon, having mind control powers is pretty damn useful. I know that I in real life would rather be able to cast Suggestion than Meteor Swarm.

Evocation: In defense of evocation. Shadow Evocation only replicates up to 4th level spells. So you'll never get contingency if you ban evocation. Or other useful spells that don't deal direct damage like Sending, Forcecage, Telekinetic Sphere, etc. And stuff that you do get, like Resilient Sphere, now allows a will save to ignore it. Also, evocation has some great damaging spells at lower levels. Sure Shadow Evocation lets you cast Magic Missile, but not with a 1st level spell slot. I think evocation is a little too heavily discounted - it actually is really good at what it does. The only problem is that any party member can deal damage, so a wizard is better off providing utility spells and battlefield control than saving the barbarian from doing some work. But it has a plethora of spells that don't allow saving throws and deal a lot of damage to a lot of people. Don't dismiss it as "just damage." Just damage means a lot of dead enemies.

Illusion: This is also one of those things you need or don't just like enchantment. Except instead of controlling their mind outright you outwit it. What I like about it is that's it's only limited by your creativity as a player.

Necromancy: Zombies slaves! Negative buffs. And a slow wizard version of resurrection, and some ways of stopping other people from resurrecting. And don't forget astral projection. But mostly zombies. I'm not a huge fan of this school. Not too much to do unless you're evil, in which case it's pretty much mandatory.

Transmutation: Everything.

--------------------

So that's my opinion at least. Personally I'd ban Enchantment or Evocation and Necromancy. Undead servants are useful but who wants to do that much rolling in battle? Plus, ya know, evil. If you're going to blast people ban enchantment, if you're going to rule people ban evocation.