Why is Julius Caesar often regarded as the greatest Roman?

Julius Caesar is considered the greatest military commander in Roman history. He showed his military genius in the eight-year long Gallic Wars, during which he conquered Gaul to the north of the Roman possession in the south (the rest of France, Belgium, Luxembourg, Holland south of the river Rhine and Germany west of the Rhine). The Siege of Alesia is considered one of his greatest military achievements and a masterpiece of siege warfare. In the civil war he fought against the forces of the senate he defeated all the military commanders he confronted.

Julius Caesar is famous for having said "Veni, vidi, vinci" (I came, I saw, I conquered).

Caesar radically reformed the state of the Roman Republic and concentrated power in his hands. he did so because with imperial expansion the Republic had become dysfunctional. The central government lost control over the provinces (conquered territories) The governors of the provinces became unruly and treated their provinces as if they were their personal fiefs. Corruption was rampant. There were military commanders who used military violence or the threat of it to get what they wanted. Caesar wanted to create a strong central government capable or restoring order in the empire.

Caesar was very popular with the middle and lower layers of Roman society. He was seen as the hero of the people who stood against an aristocracy which was seen as decadent and corrupt. This was one on the things that gave Caesar such immense power. He was a popularis. The populares (plural of popularis) were a political faction which championed the cause of the poor and wanted reforms to improve the lot of the poor. He introduced a land reform to distribute land to the poor and to retiring soldiers. He created a police force. He passed a law which wrote off 1/4 of all debts.

The most enduring legacy of Julius Caesar is the reform of the calendar. Caesar switched from a lunar to a solar calendar. Apart from some minor modifications introduced by Pope Gregory XIII in the 15th century, it is the calendar we still use.