The "topmost yoga system" is a term used by followers of Krishna consciousness, particularly in the teachings of A.C. Bhaktivedanta Swami Prabhupada, to refer to bhakti-yoga, or the yoga of devotion.
While the term "yoga" is widely understood today to mean physical postures (asanas) and breathing exercises (pranayama), its original meaning is "to connect" or "to link" with the Supreme. The Vedic scriptures, especially the Bhagavad-gita, describe various yoga systems, but they ultimately present bhakti-yoga as the pinnacle of all yoga.
Here's why bhakti-yoga is considered the topmost yoga system according to this tradition:
The Bhagavad-gita (Chapter 12) discusses the relative merits of different paths. Krishna tells Arjuna that for those whose minds are attached to His personal form, the path of bhakti is the most direct and swift. The other paths, like Jnana-yoga (the path of knowledge) and Dhyana-yoga (the path of meditation), are described as being much more difficult and arduous, especially in the current age of Kali-yuga.
Bhakti-yoga is easy to practice for everyone. It doesn't require a specific physical condition, intellectual prowess, or a secluded environment. Its core practices, such as chanting the holy names of God (e.g., the Hare Krishna mantra), hearing about God's pastimes, and offering prayers, can be done by anyone, anywhere.
The ultimate goal of any true yoga system is to connect with the Supreme and attain liberation from the cycle of birth and death. While other yoga systems can help a person realize the impersonal Brahman (the spiritual light) or the localized Paramatma (the Super-soul within the heart), only bhakti-yoga allows one to enter into a loving, personal relationship with the Supreme Personality of Godhead, Lord Krishna.
This personal relationship is considered the highest form of spiritual realization and the ultimate fulfillment of the soul's desire for love.
A true bhakta (devotee) automatically practices all other forms of yoga.
In the final, conclusive chapter of the Bhagavad-gita (Chapter 18, Verse 65), Krishna gives what is often called His "final instruction" to Arjuna:
man-manā bhava mad-bhakto mad-yājī māṁ namaskuru mām evaiṣyasi satyaṁ te pratijāne priyo 'si me
This verse translates to: "Always think of Me, become My devotee, worship Me, and offer your homage unto Me. Thus you will come to Me without fail. I promise you this because you are My very dear friend."
In this verse, Krishna makes it clear that the most direct and certain way to attain Him is through pure devotion. He re-affirms this in Chapter 9, Verse 34, and Chapter 12, Verse 8.
Therefore, according to the Gaudiya Vaishnava tradition, while other yoga systems are valid and can lead to spiritual progress, they are ultimately preliminary steps leading to the topmost system, bhakti-yoga, which offers the ultimate goal of eternal love for and service to God.