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

The question is incorrectly phrased. At the beginning of the game, everyone uses level 1 spells. And at the end, it all depends on the spells and artifacts. If a city portal, resurrection, chain lightning, or meteor shower is available at level four, then that becomes the priority. If I'm playing as Necropolis, then level three spells are the most effective. And if at level five, there's an explosion/flight/dimensional door spell, then level five is the best.
It all depends on the situation.