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If I'm playing as Necropolis, I choose Resurrection of the Dead (that's why it's my 3rd level spell). I don't like losing my warriors.
But in general, level 4 is the most useful:
Meteor Storm – powerful, deals equal damage to locally positioned units.
Chain Lightning – I just like it.
Prayer – a small speed boost allows higher-level units (4-7) to attack immediately, and it also provides a stat increase.
Berserk – a cheat spell, useful in any situation.