Best effect of the Halruuan Great Elixir

[Alleran]

For those who don't know, the Halruuan Great Elixir is an artifact potion (one use only, can only drink one per experience level) from Shining South. Consuming it can grant a whole slew of possible results (d100), some of them very bad (e.g. unwilling lycanthropy that you can't control at all for 1d6 hours every day or so, die without a saving throw, no-save feeblemind), and some can be very good (e.g. gaining spell-like abilities, ranging from mage armour and alter self to spell turning and dimension door).

One of the good ones in particular, in fact the very best one, is the maxed-out result - you immediately gain 2d4 points of either INT or CHA, and a permanent (supernatural, I think) Greater Spell Immunity against all spells of an entire school up to 8th level spells (randomly determined).

Now, assuming that a character were to gain this benefit, what would be the best school to gain this protection against? At first I thought maybe enchantment, but then I realised that you can just cast Mind Blank each day (or try and get it on an item). Conjuration, by contrast, has too many no-SR spells, so it wouldn't really make all that much difference. What about Transmutation? Necromancy? Illusion?

[The Glyphstone]

Frankly, Evocation might not be a bad choice. It's usually easier to ward against elemental energies, but this gives you Resist Infinite vs. all evocated blasting spells, including the untyped ones, and effective immunity to force damage as well. Throw up a Ray Deflection spell and you laugh at Orbs too.

[Akal Saris]

Yeah, I'd agree that Evocation would be a surprisingly strong school to ban in most games. Immunity to abjuration would mean that your buffs aren't dispel-able, however, which is very useful for a high level caster. But you could accomplish almost the same effect with a ring of counterspells.

At the very highest levels of optimization, divination might actually make the cut, since immunity to divination gives you immunity to 'scry and die' and allows you to do so with impunity to your opponents.

[Eldariel]

Abjuration, not close. Conjuration immunity doesn't actually make you immune to anything relevant, Transmutation has few annoying save-or-Xs but you can just save against those, Illusion once again doesn't play through you but the illusions themselves, Enchantment is one spell and away, Evocation is couple of spells and away (except for the stuff spell immunity doesn't protect you from anyway), Necromancy likewise. Abjuration has Dispels & Maw of Chaos; the most relevant offensive abilities for a high level character.