Perhaps I'll make many people laugh, but in Heroes 3, I always took (or tried to take) all 4 magic schools. :)
If I were to rank them in order of priority (that is, if I absolutely had to choose between them), I would arrange them as follows:
- Earth
- Air
- Water
- Fire.
However, since I don't have to make such a choice, I try to take all four schools. If I'm going to focus on magic (and I do), then I need to develop everything. Enough has already been said about the advantages of Earth and Air (I will only note that for me, the most used spells have always been mass Haste and mass Slow, and not Portal, Door, and Fly), I want to say a few words about Water and Fire.
Can you imagine what mass Confusion (Forgetfulness) is like? Especially if the battle is not with a hero's army, but with neutral creatures of one type - such as monks, medusas, mages, cyclops, wood elves, and finally - not to mention archers of the first and second levels. This reduces the difficulty of the battle many times over - and losses as well, of course. Mass Healing is generally an invaluable thing, considering that the enemy hero is usually also not idle and weakens/blinds/slows to the fullest, and the units can also stun, paralyze, poison, or curse, not to mention aging from phantom dragons. And have you tried the Mirth spell (I don't remember what it's called in the Russian version - it raises morale) on the Water Magic expert level? Everyone's morale increases by 3 points, it seems. Can you imagine an army in which 2/3 of the units move twice per turn? Believe me, this is not a small advantage.
Now, Fire. Expert Berserk covers several hexes on the battlefield, berserk on one unit is a pathetic sight compared to this. Armageddon is a great chance to use the advantage of black and gold dragons - their magic immunity. Have you ever tried moving with an army consisting only of black and gold dragons? This is not a funny option. First, the hero has fantastic movement speed on the map. Secondly, he moves first in 95% of cases. And this first move is Armageddon. If someone survives, the dragons will finish him off. Shackles of War will not be needed with such a tactic - no one will be able to escape. Thirdly, he can leave all the remaining weekly growth to guard the castle - try to take it with such forces. Fourthly, we don't lose our dragons, so the army of dragons quickly grows to frightening numbers. I'm not arguing that this tactic is not suitable as a universal one, but as an option for the Dungeon with a small distance between opponents - it worked, and very well.
And the main thing in Fire Magic is, of course, Blindness. Blindness on everyone would, of course, be cheating, but Blindness, in which a unit does not respond to an attack, is very significant! Have you ever blinded a squad of, say, 50 archangels? Or at least 30? It is possible to blind him, but it is still difficult to kill him. Here, such a trick as expert blindness - attack - again blindness was already mentioned. It's strange that there is only one mention. This is a win-win option! Of course, the fact that the hero does not have his own initiative plays a big role here. If his units do not have a turn, then he does not have a turn either.
By the way, I saw with my own eyes how, during one hot sit, the owner of the blinded unit almost had a heart attack during such a turn of the battle... His powerful squad (I don't remember which one, but it was definitely very powerful) stands there and doesn't make a single move, and he is being hit, hit, hit... And he also lost the developed hero. You can't escape from blindness.