Supreme Commander was a title bestowed upon Vorian Atreides for the supreme command of the Army of the Jihad, sometime after the death of Iblis Ginjo. Since the title is not mentioned, before, it is possible the title was invented especially for him, due to his great longevity and experience in the Jihad.
While bearing this rank, Vor initiated The Great Purge which finally trapped the Thinking Machines on one world, Corrin. Eventually the title was reformed as Supreme Bashar. It is unknown if anyone succeeded him.