Fire and Life. I play this custom faction [SHAMELESS PLUG FOR MY MOD] http://forums.elementalgame.com/447732/page/1/
Fire: Heart of Fire might be the best spell in the game. But flame dart is always useful. Focus and fireball can be awesome in the right situation.
Life: Heal is something you'll miss if you need it and don't have it. Sovereign's call is incredibly good. But Crusade and Death Ward, in the right situation, are game winners. Also, Call to Arms and wellspring can be very useful.
Getting my hands on a champion with air magic is key (air magic rocks - haste!). Also, I want to be able to cast enchanted hammers, and Mantle of Oceans ASAP, so look for opportunities to get champions with these schools.
Level 5 spheres: In my opinion, you don't need level 5 in any sphere. Life probably has the best level 5 spells. All the spheres have great level 5 spells, but none are necessary to win. I'd rather have a hero with level 3 in all schools before he got 3 level 5 schools.
Efficient level for each sphere:
Fire: level 2 is sufficient. I love going to level 3, but it's skippable. Going past level 3 is for fun only.
Life: you can get by fine with level 2, or even just level 1 (the best spell is sovereign's call at lvl 1, regeneration can heal troops).
Water: You want slow at lvl1. You want freeze at level 2. Mantle of oceans is a must-have at level 3. You might want to skip blizzard at lvl 4 - it's so amazing the game gets too easy. Also, a conclave can get you blizzard scrolls.
Air: This sphere will pay for itself with just haste, level 1. But getting to lvl 2 gets you Tutelege, which your heroes can greatly benefit from.
Death: level 1 death has such great spells, you could get by never getting past lvl 1.
Earth: I love raise/lower land, but if you are willing to give these up, you can stick with just lvl1. Stoneskin is ok, don't lament not having it. Enchanted hammers and nature's cloak are amazing spells, you want them ASAP.