If you are the greatest, the most powerful, and the highest, you are supreme.
The U.S. Supreme Court is the highest court in the land. A supreme being is a god or other higher power. A supreme leader is an all-powerful ruler. Sometimes, when everyone else is driving you crazy, you might daydream about being the supreme leader of your school, office, or maybe the entire country, so you could tell everyone else what to do. Or you might just order a supreme pizza, which usually has every imaginable topping on it.