it's a toss up between Evocation and Necromancy... the sheer power of such things as Fireball for Evocation, and the horrid nastiness of Necromancy.
The two most difficult to pull off are Illusion and Conjuration. Illusion due to the sheer amount of manipulation required, and Conjuration due to the summoning, which requires a little more paperwork to keep track of the additional creatures.
Transmutation. Things aren't the way I like them, I *make* them the way I do. Bah to Necromancy and Evocation. Who needs to raise the dead or fling fireballs when you can just warp the flesh of your enemies into whatever shape you please?
Evocation is always, without a single doubt, the worst school in both 3.X and Pathfinder. Next to that? Conjuration.
Illusion can duplicate either of those domains with a simple spell or two. Sure, it won't be as flashy and maybe not as effective, but that's just a small price to pay.
Did they nerf conjuration in PF? It's the best school in 3.5 (until high levels.)
3.5 the standard order to bar is: evocation, enchantment, necromancy, illusion/abjuration.
The reasons for this: Evocation is just bad. Everything it does, Conjuration does better.
Enchantment is gimped by immunity to mind affecting which starts coming up pretty early.
Necromancy has some strong effects but nearly all of them can be duplicated by either Transmutation or Conjuration.
Illusion and Abjuration both have a few spells you really shouldn't do without. If you must bar one of them because you want to focus on one of the first 3, you can, but it'll hurt.
Conjuration and Transmutation are too good to bar, ever. Either one of them does pretty much everything. Summons and Polymorph are the most versatile series of spells in the game. Transmutation has more ridiculous power but Conjuration is more solid in the early levels, which is where most games are played.
Keep in mind, this is 3.5 based. PF may or may not have changed the above assessment.