At lower levels, spirit is actually best, particularly with Spirit Tap.
The general rule for leveling is +damage > spirit > all. You don't want to totally neglect stamina and intellect, but spirit and damage are best. Why? The thing that slows us down most is stopping to drink. +damage = less spells needed = less mana spent = less downtime. Spirit = more mana regained from spirit tap = less downtime. For the same reason, your wand is your most important piece of equipment; you can be slack in upgrading other stuff but an up-to-date wand helps a LOT. The better your wand, the faster you kill and the fewer nukes you need.