It can be safely said that the Rosetta Stone, a document dated 196 BCE, is the most famous inscription in the world. Tons of tourists look at it in the British Museum every day, crowding the entrance to the ancient history rooms. The importance of this document and its fame are connected with the decipherment of Hieroglyphics, rather than with its primary concern, as described in the text transcribed in three languages, i.e. the royal cult for Ptolemy the 5th, a really not famous king of the Ptolemaic dynasty, which ruled over Egypt from the death of Alexander the Great (323 BCE) to the Battle of Actium, when the navy of Cleopatras and Antonius was defeated by Augustus (31 BCE).
Links
A very nice presentation of this monument by the British Museum, where the stone is kept.