This is sort of like asking, "Isn't Exxon the most beneficial brand of gasoline for human beings?" Gasoline is fuel for automobiles. Humans require entirely different fuel in order to be happy. "Spiritual" refers to the eternal self within the body. The material body needs exercise. The self can exist most naturally and happily in the spiritual world, without a material body at all.
As a form of exercise, some kinds of yoga can be very beneficial, if practiced correctly under able guidance. The same could be said for any physical fitness program. The sitting postures—asanas—practiced as "yoga" today are only the beginning stages of a yoga system from a bygone era, when people practiced for years in seclusion as a means of self-realization. But that doesn't make contemporary yoga "spiritual" in and of itself.
In the Bhagavad-gita, Krishna talks about three main divisions of yoga practice: astanga-yoga, jnana-yoga, and bhakti-yoga.
Astanga-yoga, also known as dhyana or hatha-yoga, refers to the system of sitting postures and breath control meant to bring about the peace of mind required for meditating on the Supersoul, the feature of the Supreme Person residing within each material body.
Jñana-yoga, refers to philosophical analysis of the material elements to become free from attachment to material existence. Practice of jnana-yoga enables one to merge into brahman, the impersonal feature of the Supreme.
Bhakti-yoga is the direct process of establishing a relationship of loving service to the Supreme Person, Krishna.
Krishna mentions each of these processes in His conversation with Arjuna. He then gives His opinion that since bhakti-yoga is the most direct process—it immediately puts one into direct contact with the Supreme Person, rather than after long practice of asanas or mental gymnastics—it is the best of the three.
This article more fully explains these points.
Bhakti-yoga is the relationship of loving service exchanged between a bhakta—devotee of Krishna, and Bhagavan—the Supreme Person. It is the constitutional, natural condition of our existence, but our natural inclination to serve God with love is now covered over by misconceptions about who we are (the self) and what the goal of life is. Bhakti-yoga is actually the goal of life.
In this often-quoted passage from Srimad-Bhagavatam, 7.5.23-24, the devotee-king Prahlada lists the nine main recommended practices of bhakti, any one of which can revive our natural consciousness:
"Hearing and chanting about the transcendental holy name, form, qualities, paraphernalia and pastimes of Lord Vishnu [Krishna], remembering them, serving the lotus feet of the Lord, offering the Lord respectful worship with sixteen types of paraphernalia, offering prayers to the Lord, becoming His servant, considering the Lord one's best friend, and surrendering everything unto Him (in other words, serving Him with the body, mind and words)—these nine processes are accepted as pure devotional service. One who has dedicated his life to the service of Krishna through these nine methods should be understood to be the most learned person, for he has acquired complete knowledge."
As bhakti-yoga gains popularity, so also do misunderstandings of its meaning.
Some believe that bhakti is an emotional state which can be attained through physical yoga or by chanting any kind of mantra. But this isn't supported anywhere in Vedic teachings. Anyone's natural bhakti can be uncovered through the scientific process of practicing bhakti-yoga.
Chanting the Hare Krishna mantra with attention is perhaps the single most important habit anyone interested in improving quality of consciousness can form. In principle, we should adopt any means necessary to always remember Krishna, and accept anything favorable for advancement in Krishna consciousness.
Srila Prabhupada sometimes drew parallels between chanting Hare Krishna and performing astanga yoga. For example, he would say that breath control is included within the chanting of the Hare Krishna mantra. This could be taken to mean that while chanting, one must breathe in between mantras. Such breathing is like yogic breathing because it directly provides the energy required for one to continue chanting.
However, he didn't instruct us to perform any form of yoga besides bhakti-yoga. He wisely understood the fickle and irresponsible mental condition of contemporary man. He made it clear in his books, lectures, conversations, and letters that chanting Hare Krishna alone can bring one all perfection, and no extraneous effort or adjunct yoga practice is required to attain our life's perfection.
This is not to say that we are forbidden to use our intelligence and employ whatever methods necessary to help us focus on our chanting. It is always wise to consult with more experienced practitioners when making adjustments to one's spiritual practices. With sincerity, intelligence, expert guidance, and a prayerful attitude, we can use anything in Krishna's service.