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Arguably not even all box legends are on the same tier of power.
You have the Gen III and IV box legends (and the Lake Guardians, Arceus and maybe Regigigas) who helped create the world/universe as we know it. Xerneas/Yveltal/Zygarde I guess could be considered similar since they maintain order between life and death.
Reshiram/Zekrom are said to have the power to destroy the world and embody certain traits but they didn't seemingly have a hand in creating the world or universe as we know it. Could change depending on what we learn about the original dragon in the future. (BW remakes presumably).
Solgaleo/Lunala/Necrozma are tricky. They didn't have a hand in creating the world/universe but they are very powerful and are seemingly the only beings with the power to open Ultra Wormholes. Necrozma gave light to worlds like Ultra Megaloplis and its light seemingly created Z-Power. Solgaleo/Lunala defeated the Tapu in battle and apparently gave the Tapunium Z to the Tapu, suggesting that they may be able to create Z-Power as well. On the other hand, they are also very common, much more than other legendaries and can create new Cosmog easily.