A king of the Roman Empire, in 37 B.C, became the most powerful and ruled for 44 years.Who was that king?

In 37 BC, Octavian became the most powerful man in the Roman Empire for 44 years under the titles of Augustus, which meant 'holy' and imperator, meaning 'Victorious general'. He also called himself Princeps, 'first citizen of the state'. The period of Roman history beginning with his rule upto A.D 284 is called the principate. His rule and the period following it were  peaceful and  are known in history as 'Pax Romana' which means 'Roman Peace'