I do not think any book like the Bhagavad Gita is called Ultimate over other Gitas by any Acharya as all are spiritual scriptures and so therefore holy. According to many saints, Vishnu is the Ultimate God.To many its Devi and so on. Most realized souls venerate all the forms.
If Ultimate is implied to mean most widely acceptable I think one reason of this is the early commentary by Sri Adi Shankaracharya .He while reviving Hinduism by establishing the Advaitavada, wrote a commentary of this Gita as a part of His commentaries of the Prasthanatrayi. Swami Vivekanda told:"Some infer that Shankaracharya was the author of the Gita, and that it was he who foisted it into the body of the Mahabharata."(Thoughts on Gita, 1897).He thought, however that who wrote it is unimportant. Its real greatness, which can be termed ultimacy, is in the synthesis of all the four yogas.
The commentaries of the Acharyas of all famous sampradayas and by some other famous people also made it popular. Sant Jyaneswarji was definitely one of them. I personally think that the Acharyas of different schools wrote commentaries of this Gita primarily to make the difference of their stands from that of Adi Sankaracharya clear. Swami Jagadiswarananda points out that even though there are 745 slokas in the Bhagabadgita as per Byasdeva's saying in the Mahabharata (Bhishma parba, Chapter 43, sloka 7), Adi Sankaracharya made commentary of only 700 slokas and ALL CONSEQUENT ACHARYAS also wrote commentaries of these very 700 verses and spared the other slokas (Ref: Srimadbhagavadgita, Udbodhan, page 60). I think this proves my point.
I accept Swami Vivekananda's view that this Gita is unique because of the synthesis of the four yogas-- Raja yoga, Jyana yoga, Karma Yoga and Bhakti Yoga. His Guru Sri Sri Ramakrishna Paramahansadeva has also told that the Bhagavadgita contains the essence of all holy scriptures.
But I repeat, which Gita is the 'Ultimate' would depend on the one who is reading or preaching it. All the Gitas provide highest spiritual Wisdom.
Of course, its my personal view. And so open to debate. I do not care much about what the Puranas say, because there are many versions and there is so much contradiction. The Padma Purana brands Sri Adi Sankaracharya, one of the greatest saints the world has ever seen as pseudo-buddhist and His doctrine as false. This has been put in the lips of Lord Shiva! If such puranas declare some scripture as the Ultimate, I am the last person to believe that. According to Swami Jagadiswarananda, all the other Gitas have been compsed in the 2nd Century CE immitating Srimadbhagabadgita (Udbodhan Gita, page 40). But such an opinion I think is also quite debatable.
Ref: 1.Sri Sri Ramakrishnakathamrita 2.Thoughts on Gita by Swami Vivekananda. 3. The introduction part of Srimadbhagabadgita of Udbodhan Karyalaya by Swami Jagadiswarananda.