My picks would be
- Augustus – first Roman emperor, laid the foundations that would last for over 1000 years
- Trajan – the obvious number two, who extended the Roman Empire to its greatest might and extent
- Constantine – who is best known for spreading Christianity throughout the Roman Empire, but was also a very solid military and economical ruler as well
- Justinian – known as the Restorer, he was the greatest Eastern Roman Emperor because he actually managed to bring back Western Roman lands and tried to restore Rome to its former glory
- Diocletian – while not the greatest military emperor, he initiated reforms in Roman culture and his administrative actions would keep Rome alive for the next 150 years as Rome was near collapse when he became Emperor
- Aurelian – also known as the Restorer because he saved the Empire from near collapse, by defeating every barbarian that had taken over Rome’s former territory in a short 5 year span. Perhaps Rome’s greatest military commander. Would be higher up if he ruled longer.
- Tiberius – As a military commander, Tiberius was one of Rome’s greatest, but his place on this list is marred by the gradual slip into insanity towards the latter half of his reign
- Marcus Aurelius – Known as the philosopher king, he was the last Roman Emperor to preside over the Pax Romana, a period of piece and stability within the Empire
- Antonius Pius – who wasn’t a military emperor but he presided over a great economy and a peaceful time in Rome’s history. The Bill Clinton of Roman Emperors
- Hadrian – There’s something to be said about not screwing things up. Hadrian took what Trajan built and maintained it and did a good enough job that he still ends up being on this list.