I prefer the warrior class.
And I use magic as a supplementary tool.
When playing as Crag Hack, I level up his:
Attack
Logistics/Pathfinding (whichever comes first)
Ballistics (because it's really annoying without it)
Leadership
Wisdom
Air Magic/Earth Magic
Water Magic
Intelligence (if spellcasting is really weak)/Pathfinding (or Logistics, if it's already learned)/or even better, Tactics.
Earth Magic is very useful for a warrior, mainly due to the Expert Anti-Magic skill, which can neutralize a mage's ability to cast very powerful targeted spells.
Air Magic is no less useful due to Haste (warriors need to get their stacks into battle as quickly as possible), Counterspell, and Disruption Ray. The effects of these spells are largely or completely independent of magic power (which is quite low for warriors), but they enhance the units themselves.
Water Magic is also important for a warrior. First, it can be used to neutralize (again, independently of magic power) various nasty effects like curses. In addition, it can replace high-level combat magic for the warrior. Since the warrior specializes in the strength of monsters, the Clone spell can affect the enemy just as much as an Explosion cast by a powerful mage. Fortunately, this spell is very cheap at the Expert Water level. And there's no need to talk about the usefulness of Prayer – it's a mega-skill. You can even teleport behemoths to enemy archers...
Armageddons, Explosions, Chain Lightnings – these are all child's play:p
Every warrior would benefit from having a Charm of Resistance (so as not to step on a Berserk trap, otherwise the warrior's main weapon will turn against him), as well as an artifact against Blindness... I don't even want to talk about the Sphere of Inhibition.
P.S. A mage doesn't need to level up warrior skills at all; it will only hinder him, as it will consume skill points that could be used to enhance spells. A well-developed mage only needs warriors as cannon fodder to cast as many powerful spells as possible, and at the end of the battle, he can resurrect most of his fallen units...