I really like the summoning spell (death + nature) – demonology. I’ve already written about it somewhere, but never mind. It summons creatures of death and nature, but you still need to try to acquire this skill. I would put mass buff spells for units in second place (mainly life magic). For example, dealing maximum damage with a large unit can sometimes be more useful than a single damage spell. Of course, high-level combat spells are also very powerful, but they are somewhat monotonous. Again, IMHO.
to AzteC3 and MagicMan: Can you stop arguing in every thread? And this is a forum, not a chat.