In your opinion, which primary stat is the best?

In my opinion, attack is the best stat for an abstract hero, so that, as has already been said here, they can defeat the enemy faster... but on the other hand, +1 to attack is +5% damage, while increasing knowledge by 1 at the beginning of the game significantly (sometimes by 2 times, in the case of a Warlock) increases the amount of mana, and that is much more important. Conclusion: at the beginning, knowledge is still more useful...

Added after 43 seconds
...and in general, attack.