Artifacts in Heroes III.

Of course, "Thunderclap" takes first place, allowing you to use the Thunderclap spell without spending mana.

Dum-dum-dum-dum. I disagree. On large maps, playing as a mage, you can upgrade "Lightning Bolt" to the point where it's just as effective as Thunderclap. And 600 units seems a bit weak.

Third place: "Might of the Dragon Lord" - significantly increases the hero's skills and the strength of dragons.

And it takes up almost all the skill slots...