The elephant's footprint is considered the foremost because of its size. In the same way, whatever wholesome qualities there are, they are all rooted in diligence, and they meet together in diligence; diligence is considered the foremost among the qualities.

Pada sutta - Footprint

"Bhikkhus, just as whatever kinds of footprints there are of creatures that roam the jungle, all of them are included within the elephant's footprint; the elephant's footprint is considered to be the foremost of them because of its size. In the same way, bhikkhus, whatever wholesome qualities there are, they are all rooted in diligence, and they meet together in diligence; diligence is considered the foremost among the qualities. For a diligent bhikkhu, this is to be expected - that he will develop and extensively cultivate the Noble Eightfold Path.

And how, bhikkhus, does a diligent bhikkhu develop and extensively cultivate the Noble Eightfold Path? Here, bhikkhus, a bhikkhu develops right view dependent on seclusion, supported by dispassion, based on ending, culminating in complete relinquishment; similarly he develops right intention, right speech, right action, right livelihood, right effort, right mindfulness, and right collectedness, dependent on seclusion, supported by dispassion, based on ending, culminating in complete relinquishment. It is in this way, bhikkhus, that a diligent bhikkhu develops and extensively cultivates the Noble Eightfold Path."