Śrī Caitanya-caritāmṛta Madhya-līlā 22.83 (translation by Śrīla Prabhupāda):

“The root cause of devotional service to Lord Kṛṣṇa is association with advanced devotees. Even when one’s dormant love for Kṛṣṇa awakens, association with devotees is still most essential.“

The most important five principles or practices among the 64, as Rūpa Gosvāmin states, are powerful; even just a little bit of just one of them is enough to attain pure Bhakti. But this is what I maintain, drawing heavily from the what the śāstras say: 

Let me state it emphatically, that sādhu-saṅga, among the five most important practices, is the most important, simply because it is the one practice that “awakens Bhakti,” as is stated in many places. It is not that the other practices cannot awaken Bhakti, but sādhu-saṅga must be found at the very start of this awakening process that leads to serious practice of Bhakti, in the life of the serious bhakta:

1. nāma-saṃkīrtana: Without sādhu-saṅga, how is one to know how to chant the holy names of God? Even in the prefix “sam” in the word saṃkīrtana, the connection with devotees is indicated. One receives the holy names formally from the Guru in dīkṣā. This is sādhu-saṅga. 
2. bhāgavata-śravaṇa: There is the “book Bhāgavata” and the “person Bhāgavata.” In order to truly appreciate the former, one has to have connection with the latter, and again, this is in the realm of sādhu-saṅga. One must hear the Bhāgavata from a devotee. The book Bhāgavata, additionally, is mostly about devotees of the Lord, and just by reading its text one is connecting with paradigmatic bhaktas, or sādhu-saṅga. 
3. mathurā-vāsa: Why would anyone desire to live in a holy place such as Mathurā? For greater sādhu-saṅga. What are the forces that make the holy tīrthas powerful? The śāstras say that the bhaktas make the places of pilgrimage powerful and potent. This, again, is sādhu-saṅga. 
4. śrī-mūrti-sevana: In order to worship Rādha Krishna, one must worship the devotee. Krishna is well known to have said, “One who worships me, does not worship me. But one who worships my devotee, that one worships me!” Thus Krishna comes as a devotee, Mahāprabhu Gaurāṅga. The practice of worship requires the blessings of the devotee and thus it cannot take place without the grace of devotee, as well as the loving guidance and training from a devotee. 
5. sādhu-saṅga: Because sādhu-saṅga is at the very root and at the very heart of the other four practices, it is the most important among the five most important practices. It is indeed at the start of developing premā, and it is a practice that itself deepens as one develops premā. The other four practices all require the presence of devotees in different ways. And while Rūpa Gosvāmin states that even just a little of any one of these five can impact the bhakta so powerfully, I would maintain that even such a little practice always involves some level of sādhu-saṅga. Indeed it is said that all perfection comes from even just a fraction of a moment’s connecting with a devotee. 

The challenge, then, before all of us is to appreciate all of the dimensions and aspects of connecting with a devotee: sweet friendship, profound guidance from and also given to devotees, exchanging of gifts, prasādam, and devotional secrets, understanding Vaishnava etiquette as the crown jewel of a devotee, etc.