CC Madhya 8.251

‘śreyo-madhye kona śreyaḥ jīvera haya sāra?’
‘kṛṣṇa-bhakta-saṅga vinā śreyaḥ nāhi āra’
Word for word: 
śreyaḥ-madhye — among beneficial activities; kona — which; śreyaḥ — beneficial function; jīvera — of the living entity; haya — is; sāra — the essence; kṛṣṇa-bhakta-saṅga — for associating with the devotees of Lord Kṛṣṇa; vinā — except; śreyaḥ — beneficial activity; nāhi — there is not; āra — another.
Translation: 
Then Śrī Caitanya Mahāprabhu asked, “Out of all auspicious and beneficial activities, which is best for the living entity?”
Purport: 

Rāmānanda Rāya replied, “The only auspicious activity is association with the devotees of Kṛṣṇa.”

According to Śrīmad-Bhāgavatam (11.2.30):

ata ātyantikaṁ kṣemaṁ pṛcchāmo bhavato ’naghāḥ
saṁsāre ’smin kṣaṇārdho ’pi sat-saṅgaḥ śevadhir nṛṇām

“We are asking you to tell us what is the most perfect welfare activity. I think that in this material world, association with devotees — even if it be for a moment — is the greatest treasure house for mankind.”