The Supreme welfare activity is the spreading of Krishna consciousness.

Those engaged in activities for the welfare of others are very quickly recognized by the Supreme Personality of Godhead. The Lord says in Bhagavad-gita (18.68-69), ya idaṁ paramaṁ guhyaṁ mad-bhakteṣv abhidhāsyati. .. na ca tasmān manuṣyeṣu kaścin me priya-kṛttamaḥ. “One who preaches the message of Bhagavad-gita to My devotees is most dear to Me. No one can excel him in satisfying Me by worship.” There are different kinds of welfare activities in this material world, but the supreme welfare activity is the spreading of Krishna consciousness. Other welfare activities cannot be effective, for the laws of nature and the results of karma cannot be checked. It is by destiny, or the laws of karma, that one must suffer or enjoy. For instance, if one is given a court order, he must accept it, whether it brings suffering or profit. Similarly, everyone is under obligations to karma and it reactions. No one can change this. Therefore the sastra says:

tasyaiva hetoḥ prayateta kovido na labhyate yad bhramatām upary adhaḥ (Bhag. 1.5.18)

One should endeavor for that which is never obtained by wandering up and down the universe as a result of the reactions of karma. What is that? One should endeavor to become Krishna conscious. If one tries to spread Krishna consciousness all over the world, he should be understood to be performing the best welfare activity. The Lord is automatically very pleased with him. If the Lord is pleased with him, what is left for him to achieve? If one has been recognized by the Lord, even if he does not ask the Lord for anything, the Lord, who is within everyone, supplies him whatever he wants. This is also confirmed in Bhagavad-gita (tesam nityabhiyuktanam yoga-ksemam vahamy aham). Again, as stated here, tapyante loka-tāpena sādhavaḥ prāyaśo janāḥ. The best welfare activity is raising people to the platform of Krishna consciousness, since the conditioned souls are suffering only for want of Krishna consciousness. All the sastras conclude, therefore, that spreading the Krishna consciousness movement is the best welfare activity in the world. Because of the ultimate benefit this bestows upon people in general, the Lord very quickly recognizes such service performed by a devotee.

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Eighth Canto, Chapter 7 – Text 44