The Supreme System Lord was the title that the Goa'uld System Lords afforded to Ra. The title was an acknowledgement of Ra's authority, power and cunning and it allowed him to be recognized as the nominal Emperor of the Goa'uld and the leader of the Goa'uld Empire, though the other System Lords continued to try to overthrow Ra.
It is known that Sokar once ruled the Goa'uld, but if he held the title of Supreme system lord is unknown.
No other Goa'uld ever claimed the title after Ra's death at the hands of Colonel Jack O'Neill and Dr. Daniel Jackson of the Tau'ri on Abydos in 1996. (SG1: "The Tok'ra, Part 1") However Ba'al did attempt to claim the position while secretly serving Anubis behind the scenes. (SG1: "Threads")