The most authentic commentary of Bhagavad Gita that contains the original spirit of Bhagavad Gita is available with Gita press, Gorakhpur, India.
Nothing in this world paralleled the publications of Gita press, Gorakhpur, India.
I have no personal affiliation with Gita press in any manner. I have gone through their publications in depth and also from other renowned publishers of India. The publications of Gita press are not only highly subsidized; they were the most authentic by authors like Jayadayal Goyandka, Swami Ramsukhdas and Hanuman Prasad Poddar.
The aim of Gita press is to publicize and provide essence of spirituality to one and all world over and that too at very reasonable prices. Almost all publications of Gita press are sold below manufacturing price. The most authentic commentary of Bhagavad Gita is available for mere $3.
If one wants to indulge in commentaries by Adi Shankaracharya (an enlightened one), commentary on Bhagavad Gita and also 11 principal Upanishads (independent spiritual treatises) is available with Gita press.
I personally advise going to a Gita press store in India which are located in most major cities. Gita press also has an online store. One can choose a translation or a commentary as suits our palate and thinking process.
However, the most authentic commentary on Bhagavad Gita comes from Lord Krishna himself. My pursuit of God started when six years of age. Until reaching God 37 years of age I did not indulge in Bhagavad Gita even for once. Rather I did not open Bhagavad Gita not for once. Yet, everything contained in Bhagavad Gita was within my reach. How?
From six years of age my faith in God Almighty was absolute 100%. I also believed in the world of souls atmans. I walked path of truthfulness 100%. The moment I demolished my ego to zero around six and half years of age, I could distinctly hear promptings of Lord Krishna coming from within my heart. And thus started my actual pursuit of spiritual life!
All my life from six years of age I could distinctly hear Lord Krishna guiding me from within. Everyone can duplicate this path by indulging in contemplation (chintan) and path of truthfulness.