[WraithLord]

I voted for astral.

has some nice SC buffs - astral weapon/shield, body ethereal (4 early game), luck and very important twist fate.
Has some strong battle field spells such as doom, enslave mind, astral tempest.
Good summons (imo), I always find the harbringer and the angels good usage. With the right items and spell scripting they can be used as SCs. especially vs. death/blood thugs. And let's not forget abomination.
And good rituals useful for spying, assassination.
And last not least is wish.
I find it to be a complete spell school.