What are you going to be soloing and how? Undead and using nukes and dots? Normal mobs and letting them die on DS and Reverse DS, while HoTing yourself? Some other mob / tactic?
Most of your spells will be alteration, including rezzes, heals, hots, cures, pacifies, fears, transport, and some others. Primarily buffs are abjuration; nukes, stuns, undead dots are evocation; and summon spells are conjuration. Divination is miscellaneous sense type spells.
You definitely don't want Divination or Conjuration, and it's unlikely you'd want Abjuration unless you were going to be a buff box. So your pick is Alteration (about 75% or more of your spells), or Evocation. Personally, I'd go with Alteration no matter how I played, since heals are your primary focus, and making them better only improves your performance, and gives the benefit to the widest range of spells.