Yet another important element to this is how you field your caster: Mounted on a monster, joining a unit or as a lone character. Lone characters benefit from a 360° vision arc, which is especially important if you want to buff your units. Casters joining a unit are bound by the units vision arc, so chances are high that your caster could only boost his own unit. And also he’s likely to get into combat, so you need something with a good assailment spell. A caster on a flying monster has a high degree of movement, but is also bound by his forward arc, but this can me mitigated a bit with the free 90° lumbering move. Let’s see in which areas the three lores are strongest and how which are therefore the best fits for the three types of casters.
Daemonology: With the exception of the Vortex of Chaos (which is still fine), all spells in Daemonology are really good. For me, it shines particularly in Enchantment, Hex and Conveyance.
Elementalism: The weak part is assailment, the other ones are quite good, especially Hex, but also Conveyance and Enchantment.
Dark Magic: Overall I’d consider it the weakest lore, but Hex and Vortex are interesting.
Caster on monstrous mount: With this build, your caster will likely be on his own, flying at the flanks or in front of the main army, usually facing the enemy. Especially in case of a Bale Taurus, this caster will likely see combat! With this build you do not aim at boosting your army. We’re looking for particularly potent hex, missile, vortex and assailment spells. From our available lores, I’d say Elementalism is the most useful one in these categories. Storm Call and Plague of Rust weaken the enemy, the vortex blocks line of sight and Wind Blast has S5 and pushes back the enemy. From our signature spells you could go for Ash Storm which, thanks to your monstrous mount, you may be able to cast from behind enemy lines and affect multiple units. or Curse of Hashut against T3/4 armies. The assailment spell is not very good though. Word of Pain and Phantasmagoria from Dark Magic are interesting too, but overall I find Dark Magic the weakest of the three lores. Daemonology is tricky. On a Bale Taurus you could go all in with the Enchantment and buff yourself into a truly fearsome fighter, especially with Daemonic Vessel.
Caster in a unit: The most restricted in terms of vision arc. Enchantments and Conveyance spells can most likely only be cast on his own unit. Daemonology probably takes the cake with its two excellent enchantment spells (Daemonic Vessel can even be cast when the unit is already in combat) and Steed of Shadows. And the assailment spell is better too. Unfortunately our signature assailment spells is quite weak, might be better served with Ash Storm or, situationally, Curse of Hashut.
Lone character (bonus point for Arabyan Carpet): Daemonology and Elementalism again, it’s really a toss-up. Daemonology’s Daemonic Vessel is lost, but both lores are still very good at giving a unit a crucial buff or taking away something from the enemy.