Tengwar is the script used to write the One Ring inscription in J.R.R. Tolkien’s world.
The inscription appears on the Ring in fiery letters when the Ring is heated. It is written in Tengwar, a script Tolkien designed for the Elvish languages and later adapted for other languages of Middle-earth. The lettering is therefore a writing system, not the name of the language spoken by the inscription.
The words themselves are in the Black Speech of Mordor: “One Ring to rule them all, One Ring to find them.” This distinction is a frequent source of confusion. Tengwar identifies the characters on the page or object, while Black Speech identifies the language being written.
Cirth is another Middle-earth writing system, especially associated with Dwarves and with inscriptions. Sarati is an earlier script in Tolkien’s invented writing history. Angerthas refers to rune arrangements derived from Cirth, so it is not the best answer for the Ring inscription.