What is the superlative form of heartily?

There isn't a superlative form of "heartily" in the traditional sense. "Heartily" is already an adverb expressing a high degree of something, like enthusiasm or enjoyment.

To express an even higher degree, you could use:

* Most heartily: This emphasizes the intensity of the action.

* With the greatest heartiness: This is a more formal way to convey a high degree of enthusiasm.

* With all your heart: This emphasizes complete and sincere dedication.

For example, instead of saying "He laughed heartily," you could say:

* He laughed most heartily.

* He laughed with the greatest heartiness.

* He laughed with all his heart.

Ultimately, the best option will depend on the specific context and the level of formality you are aiming for.