He pretty much invented the position, overseeing the transition from the failed Republic of Rome to the quasi military dictatorship of Emperor Rule. Augustus was a brilliant politician, he was strong while still yielding to the Senate and his rule was one of the most peaceful and prosperous in Roman History, called the Pax Augustus

He is best known as the first Christian Emperor, the man who converted Rome to Christianity and convened the Council of Nicaea, the first great council of the Church. However, Constantine is surpassed only by Augustus in political acumen and importance. He saved the collapsing Empire at a time of great strife and thanks to his wise lead it continued on for another thousand years in one form or another.

Like Augustus and Constantine, Vespasian saved the Empire at a time of great chaos. After the disastrous rule of the Year of the Four Emperors Vespasian's steady hand and no nonsense policies helped save the Empire from financial ruin and set it back on the right track.

The success of his rule can be questioned, The Roman Empire was not as peaceful or prosperous under hsi rule as some others, but Marcus Aurelius is one of the most famous Emperors largely due to his scholarship and intelligence. One of the most famous Stoics in history, Marcus Aurelius wrote a great deal of philosophy and was known as a great, moral man and fine thinker. His contributions to literature and philosophy are the greatest of any Emperor and his writings are still studied today.

Technically, an emperor of Byzantium, but that was the successor to Rome, and in his short reign he took Italy back for Rome. If his treasury hadn't been so limited, he most likely could have restored the Roman Empire.

Many scholars believe that while Constantine gets all the credit for renewing Rome, it was actually Diocletian who was responsible for saving the Empire after it started to deteriorate from wars, rampant spending and poor rulers.