* rex (masculine), regina (feminine): This refers to a king or queen, a ruler of a kingdom.
* princeps: This word originally meant "first citizen" but came to be used for a ruler, especially in the Roman Empire, often referring to the emperor.
* imperator: This word specifically refers to a Roman emperor, a military commander with supreme authority.
* dux: This word means "leader" or "duke," and could be used to refer to a ruler, particularly a military leader.
* gubernator: This word means "governor" or "pilot" and could be used to describe a ruler in some contexts.
The best translation will depend on the specific type of ruler you're referring to.