In your opinion, which primary stat is the best?

The line between defining importance is very thin, but let's try to determine: strength is always needed, as you always attack with it, just like magic, because powerful spells can decide the outcome of a battle against a clearly weaker army!!! Defense is necessary in a prolonged battle with mutual bloodshed, and in this case, losses will still occur, i.e., the best defense is offense!!! Knowledge – I usually approach major battles with full mana, and with good magic and attack, I think not much mana will be needed, especially since a hero can take out powerful enemy units; he usually manages to take out at least one:
Attack
Magic
Defense
Knowledge
Of course, this does not mean that attack should be twice as much as the rest; everything should be balanced, and usually it is!!!

Added 1 minute ago

Knowledge... you can't do without it.

A lot of mana without magic is useless, unless you're just taking warriors from the Castle!!!))))