[Endoperez]

All four elements: Conjuration gives Earth/Phoenix/Storm/Water power, that give +1 to caster level. Conjuration also lets you summon smaller, then bigger elementals, and eventually the elemental royalty (Kings of Fire and Earth, Queens of Water and Air).

Fire: good at Evocation. Most summoned units aren't that powerful, but Fire Snakes are nice. Fire and Death make a good combination, because that allows you to make a cheap Fire booster Flaming Skull (Constr 6).

Water: mixed goodness. Quickness from Alteration spells, Boots of Quickness (Wat2) and Jade Armor (Wat1 Earth1) makes tough units doubly good. Evocation has Falling Frost and few other nifty damage spells, but they're mostly from level 5 up. There's Frozen Heart high up in Alteration, Sailor's Death in Thaumaturgy, and some nice summons in Conjuration. Water/Fire mages can use Acid line of Evocations, which are quite good and easy to research.

Earth: Good at Alteration (armor buffs, armor destruction, Earth Meld, Petrification!), at Evocation (Blade Wind, Earthquake and multipath spells Rain of Stones, Gifts from Heaven, Magma Eruption), and it's one of the easiest paths to boost, with Earth Boots at Construction 4 and Earth Power available in all battles.

Air: Has some buffs Alteration (Mirror Image, Mistform, Phantasmal Army, Mists of Deception) and Evocation (Lightning Bolt, Thunder Strike etc) spells. Storm (Evoc) blocks most flying and most missiles, which can be very good. Air gives cheap Flying Boots, and has teleport spell Cloud Trapeze in Enchantment. Seeking Arror, in Enchantment, is good when massed against human mages. Chain Mail of Deception is nice armor for fighters, and if you get an Air 4 mage he can forge two magic boosters (Barrel of Air and Winged Helmet).

Death: good in Conjuration (Bane Lords, Wights, Wight Lords; even Black Servants can be useful if you give them cheap armor and weapons, and the tougher commanders are much better) and Enchantment (Reanimation is bad because it takes gems, but Raise Dead/Raise Skeletons are very good battle spells when massed. Behemoths are nice. Reanimate Archers is good if you can cast it.) For combat spells, you can research Terror, Rigor Mortis or Life After Death from Thaumaturgy, Shadow Bolt/Blast from Evocation, or Drain Life/Soul Vortex from Alteration. Death and Fire let you cast Banefire, and summon King of Banefires, a former King of Elemental Fire who has been corrupted by necromantic magic.

Astral: Thaumaturgy all the way! Mind Burn, Soul Slay, Enslave Mind... Evocation lets you kill commanders from afar, with Mind Hunt. Just make sure there are no enemy Astral mages in the province. Alteration buffs Body Ethereal and Luck can be cast on nearby friendly enemies, which make them much more survivable, which is very good when you have tough units.

Nature: Enchantment has Charm(!!), Gift of Health (!!!) and other nifty spells. Conjuration has lots of summons, but most aren't very good, and Tangle Vines and Maggot, which help a little but not much. There are some Evocations. Nature items often give regeneration, berserk and reinvigoration (=removes fatigue), which are good in fighting commanders, although berserk can be a double-edged sword.

Blood: Blood is different. Very different. If you want to try it, go for the fliers as Devils fast as you can. Fiends of Darkness, Devils/Frost fiends, etc. Dark Vines are tough, Demon Knights hit very hard. And especially in single-player, where there isn't any competition, you get lots and lots of unique commanders that can be equipped to destroy whole armies: Ice Devils, Arch Devils, Heliophagii... the Demon Lords are good, but perhaps not quite at the level the spell's Blood requirement would make you think. They are very nice mages though.