Which level of magic do you use the most (Heroes of Might and Magic III)?

In my opinion, the main thing is to become an expert in a specific area of magic (earth, air, fire, water)... and then just spam mass spells. The simplest spells (blessing, slowing, hastening, healing...) play a very important role when you can cast them on everyone at once. And those same spells of a higher level (chain lightning, berserk, explosion, portal, dimension door...) often come in handy when mobility on the map is needed or when the hero's army has a clear numerical and strength advantage.