what is the most important thing that julius caesar accomplished
This is a very difficult question to answer because it is entirely subjective. Julius Caesar was a brilliant politician and military strategist. That, in itself, is a rare blend especially in the ancient world. To take a modern example, Ulysses S. Grant was considered by many to be a military "hero" for his leadership during the American Civil War. But, once elected president, he placed many of his cronies into positions of power and influence and is remembered by many historians as a failure as a president, though he was a highly principled man. It is not easy to transition from a military career into politics.
Another significant contribution that history remembers Caesar for is his ethnographic history of the Gaulish people in the first century B.C. There have been authors since Caesar's time who have written similar accounts, such as Ammianus Marcellinus during the fourth century A.D., but none have quite equalled Caesar's Gallic Wars. I think a comparable comparison could be made with Herodotus' Histories, an ethnographic snapshot of the ancient world. It is interesting to note that Herodotus, too, was a general like Caesar, and many historians have bestowed Herodotus with the title "Father of History."
But, as many will argue, Caesar may be best remembered for initiating one of Rome's greatest eras of conquest and expansion while also consolidating immense power and prestige during the First Triumverate, ultimately leading to a weakened senatorial class and the fall of the Roman Republic. After Caesar's assasination, it would be his adopted nephew who would rule ruthlessly as Caesar Augustus for decades, cementing his position and power while also expanding his empire.