The dictionary says a ruler is the one who rules, specifically a sovereign. Or a worker or machine that rules paper. It could also be a smooth-edged strip of wood or metal that is usually marked off in units and is used as straight edge for measuring. This word debuted in the 14th century.
While the word supreme debuted in 1513, from latin supremus or superior. It came to mean the highest rank,of authority, highest in degree or quality the ultimate.
That would mean a Supreme Ruler would either be totalitarian, or one who is of the highest rank. With the advent of democracy the plebeian has become the sovereign, the government is ourselves, not an alien power over us. The ultimate rulers are we the people and not the president or the priminister! unfortunately As Damon Meredith rightly says, the world is ruled by cowards and cravens: brave men have put them there.
Thats not Plinky enquired though, what would I do if I were the supreme ruler, simple I would shut down all structured school and make it a criminal offense to send your kids to all those extra courses. No child should be sent into a school room even if it kindergarten nursery, pre-school, play-school whatever till the age of six and formal writing would only begin at the age the 8.