The most important spells in the game, by far, are clerical healing and protection from magic spells, plus the mage transportation spells.
A small step back are some of the light protective spells. Air: Wizard Eye, Invisibility, Fly Fire: Torchlight Water: Awake, Town Portal Body: Heal, Regenerate, Cure Poison, Cure Disease, protect from magic Mind: Cure paralysis, Cure Insanity Spirit: Remove curse, Raise dead Light: Dispel magic, Day of Gods, Hour of Power, Day of Protection
You will see that no offensive spells are listed here. Those are really up to your preferences. However, IMHO, offensive spells seem to play a much lesser role in MM8 than they did in MM7. Back in MM7 fire spells like meteor shower and inferno were vital. Pain reflection was an awesome spell. Etc. Somehow, with MM8, past the mid game magic always plays a lesser role than I'd expect. By midgame, I trade my mage for a dragon, and let my Dark Elf handle the need for town portal, etc.
Just an opinion!