Nichiren gives several reasons why Shakyamuni Buddha is great

"Reply to Yasaburo" 

" I SUGGEST you begin the religious debate with these words:] “Although I am an ignorant layman, among the teachings I have heard, I was especially impressed by the passage in the second volume of the Lotus Sutra that reads, ‘Now this threefold world [is all my domain . . . ]’”1 This sutra passage means that this country of Japan is the domain of Shakyamuni Buddha. In addition to the fact that all the gods, such as the Sun Goddess, Great Bodhisattva Hachiman, and Emperor Jimmu, and all the people, from the ruler of the nation on down, dwell within his realm, he is the Buddha to whom we living beings are greatly indebted for three reasons. First, he is our sovereign; second, he is our teacher; and third, he is our father. Among all the Buddhas of the ten directions, only Shakyamuni Buddha is endowed with these three virtues. Therefore, even if all the people of the country of Japan were to serve Shakyamuni Buddha wholeheartedly just as they now do Amida Buddha, if they were to place him side by side with another Buddha and treat him in the same manner, that would still be a grave error. For example, though someone were our own ruler and wise besides, if we were to shift our allegiance to the ruler of another country—for instance, if, while dwelling in Japan, we were to pay honor to the king of China or Koryŏ and slight the sovereign of Japan—could we be called persons who honor the great sovereign of this country?"