In general, all elements are useful; in different situations, some may be more useful than others.
For Warriors, the most useful spells are definitely:
haste, slow, bless, bloodlust, and blind – arranged in descending order (for battles with humans or the AI);
slow, bless, haste (for neutral creatures)
For Mages, chain lightning and explosion, resurrection and frenzy, elemental summoning, and armageddon are good...
This only concerns combat spells... and there are also flight, gate, and portal...
And what is there to discuss about what is more important? Unless it's about which element has more useful spells and is used more often...