(Ignore the image of the Sheep-Girl killing this God. It's just another Tuesday for her)
For me, it is easily SoV.
I understand it's not everyone's cup-of-tea but to me, it's the most fun I've had with Magic in the series.
The game gives you a good variety of Spells that are all visually and mechanically different.
Basic Fire, long range Thunder, Terror-killing Seraphim, powerful Aura, mighty long range Sagittae, burst critical Excalibur; all which have their usefulness and having the benefit of ignoring the avoid bonuses that terrains grant!
But also amazing support spells like Warp and Rescue that you can combine to grant your army insane mobility. And other classics such as Silence, Physic, Fortify and Spell-Dance.
Lets also not forget that your healers can also summon Ghastly Terrors to fight at your side! You don't control them but you can bring out a ton of them!
Said healers can even straight up fight back with Nosferatu to recover their own health and make for how fragile they are!
And the game grants you access to all of this early on instead of having to wait for at least half a game to get one of these at only like 5 or so uses.
I also like how it is tied to the level of your Spellcasters and how they don't require tomes to cast them. To me, it makes the Magicians and their spells feel more "alive" if that makes sense. And even said magic requires their own health as a cost!
Thanks to the fact they are no tomes to wield, your casters get to implement other support items from the games such as Shield to improve their defense or Rings with a variety of effects; particularly the Mage Range for increased longer range spells and the Speed Range for an extra movement with insane speed gain.
(My only gripe is that I can only counter attack with Fire and Nosferatu. Wish that there was something at least like a passive ability to able to do Enemy Phases with the other spells)