The oldest title

Pontifex. The office of Pontifex Maximus — “greatest priest” — was created in the 8th Century BC by Numa Pompilius, the second king of Rome (and probably the only Roman leader ever who was something of a pacifist). The title has been in continuous use ever since. Mike Duncan, The History of Rome Podcast, episode 3:

The office of Pontifex Maximus would remain an independent authority until the title was claimed for the emperors by Augustus at the dawn of the Imperium. At the end of the empire, the title was passed to the popes, who hold it to this day, 2600 years after its inception.