Technically, as the last ruler of Rome’s Republican era, Gaius Julius Caesar was never recognized as an emperor. But it’s impossible to tell the story of Rome (or its eventual transition from a republic to an empire, without mentioning Julius Caesar. Aside from being a successful general, conquering Spain and Gaul—feats that greatly expanded the size, power and wealth of Rome—Caesar enacted a number of foundational reforms that would set up the oncoming Roman Empire. As leader of the Roman Republic, Caesar increased the size of the senate to represent more Roman citizens, established the Julian calendar (the 365-day, 12-month calendar still in use worldwide), granted Roman citizenship to all those living under Roman rule and redistributed wealth among the poor.

Gaius Octavius Thurinus, also known as Octavian or “Augustus,” served as the first official emperor of the Roman Empire, and is often seen by historians as the greatest. The emperor (for whom the month of “August” is named) introduced the period of peace known as the Pax Romana, which saw the Roman economy, agriculture and arts flourish. During that period of relative peace, Augustus also established a number of reforms—including tax incentives for families with more than three children and penalties for childless marriages—that helped the Roman population grow. An aggressive builder, he also oversaw the construction and rehabilitation of many of Rome’s great temples and the strengthening of its legendary aqueduct system.

In ancient Rome, few emperors were better at acquiring land for the empire than Tiberius Caesar Augustus. Rome’s second emperor owes his place on this list solely due to his military conquests. As an emperor and politician, Tiberius is largely considered to have been uninterested in the job and not shy in showing that disinterest. (Roman philosopher Pliny the Elder called him “the gloomiest of men.”) When it came to conquering neighboring lands and expanding Rome’s territory, however, few were better. During his reign, he oversaw one of the greatest military expansions in ancient Rome’s history, widening the empire’s boundaries deep into present-day Croatia and Germany.

After the tyrannical reign of Emperor Nero, Rome found itself in a crisis of instability. So much so that during the 12-month span following Nero’s death, the empire had four different rulers (known as the “Year of the Four Emperors”). It wasn’t until Titus Flavius Vespasianus ascended the throne that stability and prosperity returned to Rome, setting the nation back on track. During his reign, Vespasian helped reform the financial system and began many ambitious construction projects, most notably the Colosseum. Vespasian was also the first Roman emperor ever to be succeeded by his son. That father-son handoff would lay the groundwork for the Flavian Dynasty, a near three-decade period of fiscal and cultural prosperity.

Often in the conversation for “greatest Roman emperor” by historians, Marcus Ulpius Traianus was the second Roman emperor in the Nerva-Antonine dynasty commonly referred to as Rome’s “Golden Age.” Bolstered by one of the greatest military expansions in Roman history, Trajan’s reign marked the peak of Rome’s geographic expansion, as it covered nearly 1.7 million kilometers of territory in Europe, Africa and Asia and boasted nearly 57 million people. In addition to his military successes, Trajan also oversaw many ambitious building projects, including the still-standing architectural marvel, Trajan’s Column. He also expanded Augustus’ financial aid programs for poor Roman citizens, in turn providing one of the earliest examples in history of a federal welfare program.

Publius Aelius Hadrianus claims a spot as one of Rome’s most influential emperors for his ability to secure Rome and its borders and the unprecedented engineering prowess he displayed while doing so. He oversaw construction of Hadrian’s Wall, a 73-mile-long defensive fort—much of which still stands today and is recognized as a British cultural icon. He also leaves the Pantheon, which revolutionized architecture with its innovative construction of shapes built with concrete.

Titus Aelius Hadrianus Antoninus Pius presided over Rome during one of the civilization’s most peaceful periods. That lack of turmoil afforded Pius the opportunity to focus on advancing on the infrastructure successes and civic reforms of his predecessor Hadrian. His greatest contribution to Roman civilization, however, came through the legal system. As the first Roman emperor to adhere to the concept of “natural law,” Pius instituted a legal system that would serve later as the reference point for many nations developing their own legal systems, including Britain, France and Germany.

Known as the “emperor-philosopher,” emperor Marcus Aurelius Antoninus produced writings now considered philosophical canon. A fervent adherent of Stoicism—a Hellenistic school of philosophy that claimed that becoming a clear and unbiased thinker was key to gaining universal reason—the emperor (who was famously portrayed in the Oscar-winning Gladiator) is widely regarded as one of history’s most essential philosophers. His book Meditations is largely regarded as a literary masterpiece.

Publius Licinius Valerianus makes the influential list less for what he did than what was done to him. In 260 A.D., after the Battle of Edessa against the Persians, Valerian (a notorious persecutor of Christians) became the first Roman emperor to be taken as a prisoner of war. The unprecedented capture sent shockwaves through the Roman Empire, only to be exacerbated by the fact Valerian was never rescued. The emperor went on to die in captivity under unknown circumstances. Rome’s inability to rescue its own sovereign would deal a seismic blow to the mystique of power the Romans held over the world. And, many historians believe, it would plant the seed in the minds of foreign nations that the previously “unconquerable” nation of Rome could indeed be toppled.

On one hand, Gaius Aurelius Valerius Diocletianus deserves to be remembered for saving Rome from the “Crisis of the Third Century,” a near 50-year period—marked by civil war, political instability rebellions and invasions—during which the empire nearly collapsed. On the other, some historians believe it was his installation of the “tetrarchy” form of government that might prove his most valuable contribution. Under the tetrarchy, Diocletian mandated that Rome would be ruled by four leaders: an emperor in the west, one in the east (the “Augustus” emperor) and two junior co-emperors (the “Caesars”). The tetrarchy didn’t last, but it did provide the groundwork for the practice of splitting the Roman empire into eastern and western halves, a move that would prove crucial in extending its lifespan.

Considered by many to be the last western Roman emperor, Constantine I brought many changes that would irrevocably alter the Roman empire. He was the first Roman emperor to convert to Christianity and permanently established religious tolerance for Christianity with his Edict of Milan in 313 A.D. Constantine also built Byzantium (later renamed Constantinople), which would become the empire’s epicenter for the next thousand years and mark the transition into the new epoch known as the Eastern Roman Empire or Byzantine Empire.