Heart was the most powerful ring from Captain Planet, but Gaia knew that only Ma-Ti was mentally strong enough to never abuse it.
It allows one to change the heart and mind of an individual.
Imagine that power in the wrong hands. It could be used to manipulate anyone. It's effectively an OP charm ring that could allow you to manipulate your way into taking over the world.
However, Ma-Ti would never. He knew how to be humble and understood the gravity and power of the ring.