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

It all depends on the situation: in the early stages, use the first level, then the second. The third and fourth levels are only for combat spells and those that allow you to win a battle much later. The fifth level is for large maps, when you get tired of running around, and you use portals, dimensional doors, etc., and in battles – summon elementals and use the explosion spell.
But still, use them less often than the first and second levels – they are more versatile, especially at the beginning of the game, as you develop your hero for these two levels: if you have speed boosts, aim for an expert in air magic; if you have slowing effects, aim for earth magic, etc.

Sincerely, Draco