I voted Abjuration. No matter what kind of Arcanist you are, you need shield, dispel magic, globes of invulnerability, protection from elements, mind blank, etc.
I don't think that proves that WotC messed up too badly however. A school being useful is not the same thing as a school being useful to specialize in. While, I think that on the whole abjuration is more essential than transmutation, conjuration, or evocation, a free slot and half of my free spells coming from any of those schools would be far more useful than a free slot that has to be used for Abjuration and half of my character's free spells being abjurations. An extra glitterdust would usually be more useful than protection from arrows. Similarly, an extra haste or fireball would often be more useful than dispel magic. (Although the dispel magic is a pretty useful slot spell) It would certainly have been nice to pick up Cat's Grace as a second level spell instead of Resist Elements (but that wasn't an option as an abjuration specialist).
If you ask me, the priority of schools sacrificed and of school specialty should be different.
Abjuration should be a tier 1 sacrifice--giving it up is as difficult as giving up evocation, transmutation, or conjuration. On the other hand, it's accurately placed as a tier 2 specialization. It's not as good to specialize in as the top tier schools.