Supreme Sorceress was the title used by the leader of the Sorceresses of Rossak prior to the establishment of the Bene Gesserit. Probably the most famous was Zufa Cenva, who led the women of Rossak during the Butlerian Jihad.
After Zufa's sudden death the title passed briefly to her daughter, Ticia Cenva. When Ticia died on Rossak toward the end of the Jihad, the title unofficially went to Raquella Berto-Anirul, an off-worlder who led the Sorceresses off Rossak and founded the Bene Gesserit order on Chapterhouse.
Obviously the title was later understood as Reverend Mother after the Bene Gesserit reformation.