We know what it means to be a “Cursed Sword”. It means that the sword is a strong sword and will not tolerate a weak user. Since Law’s previous sword was a 50 Skilled Grade, it naturally follows that Kikoku must be stronger than that, so on par with a 21 Great Grade
We also know what it means to be unranked. It is like Kaido’s Kanabo where the strength of the user is what determines its true worth, its “grade”
Kikoku is Law’s way of being a swordsman yet not conforming to this idea of “the stronger you are the stronger your sword must be.” Kikoku is deliberately kept ambiguous so that its worth grows in conjunction with Law, because his fruit is already worth more than any other Meito or Saijo out there