What are The Buddha's most compassionate acts? Source could be from sutras or anything else. Thank you.
Probably the decision to become a samma sambuddha for the benifit of others. Becoming an arahath and ending your own suffering is lot more easier and take lot less time.

“Then I listened to the Brahmā’s pleading, and out of compassion for beings I surveyed the world with the eye of a Buddha. Surveying the world with the eye of a Buddha, I saw beings with little dust in their eyes and with much dust in their eyes, with keen faculties and with dull faculties, with good qualities and with bad qualities, easy to teach and hard to teach, and some who dwelt seeing fear and blame in the other world. Just as in a pond of blue or red or white lotuses, some lotuses that are born and grow in the water thrive immersed in the water without rising out of it, and some other lotuses that are born and grow in the water rest on the water’s surface, and some other lotuses that are born and grow in the water rise out of the water and stand clear, unwetted by it; so too, surveying the world with the eye of a Buddha, I saw beings with little dust in their eyes and with much dust in their eyes, with keen faculties and with dull faculties, with good qualities and with bad qualities, easy to teach and hard to teach, and some who dwelt seeing fear and blame in the other world. Then I replied to the Brahmā Sahampati in stanzas:
‘Open for them are the doors to the Deathless, Let those with ears now show their faith. Thinking it would be troublesome, O Brahmā, I did not speak the Dhamma subtle and sublime.’
MN26