Nichiren on the superiority of the Lotus Sutra among all sutras

"All these passages resemble the one from the Lotus Sutra that speaks of the past, present, and future. But such passages are declaring that a certain sutra is a “king” in comparison to the sutras of Brahmā, Shakra, or the four heavenly kings, or that it is a “king” in comparison to the Hinayana sutras, or that among sutras such as the Flower Garland or the Shrīmālā it is the most outstanding. They are in no way saying p.483that that particular sutra is the king of kings among all the Mahayana and Hinayana, provisional and true, exoteric and esoteric sutras preached by the Buddha during his entire fifty or more years of teaching. It is important that we should clearly understand what is being compared when we try to decide which sutras are superior and which inferior. Only when one has succeeded in overcoming a powerful enemy can we say that one has displayed real strength.

Moreover, the relative superiority of those sutras [that claim each is “king”] is determined by a single Buddha, Shakyamuni, and is not supported by the words of testimony given by Many Treasures Buddha or the emanations of Shakyamuni Buddha. One must not confuse a personal opinion of one Buddha with the public announcements in the Lotus Sutra verified by all the Buddhas." - Choosing the Heart of the Lotus Sutra

This is but one argument set forth by Nichiren. Nichiren gives even more elegant reasons in his five major works, particularly, the Opening of the Eyes, The True Object of Devotion, and On Repaying Debts of Gratitutude. In the Opening of the Eyes, he cites the fact that the other sutras reject certain people from attaining Buddhahood, those who carried out five cardinal sins, women, or the Men of Learning (Sravakas) and Self Realization (Pratyekabuddhas) and they assert that the Buddha attained Enlightenment for the first time under the Bodhi Tree. The Lotus Sutra teaches that through its faith and practice, even those who carried out the five cardinal sins, women, Men of Learning and Self Realization will all attain Buddhahood. Most importantly, only the Lotus Sutra reveals that Shakyamuni Buddha attained Buddhahood in the eternal past and has been preaching and saving the masses of beings since time immemorial.