His military accomplishments are hard to deny or over-estimate. His contribution to the loss of the Roman republic is unfortunate. It's impressive that he was not only a great general and political leader but also an accomplished writer.
This question is really a matter of opinion and there will be many different answers. In my opinion, the greatest accomplishment that Caesar won in his life was his great military victories. I am not stating that his military accomplishment were all just. For example, his actions against his own people was questionable. But there is no question that Caesar was one of the greatest generals that ever lived in the history of the world. Here are few of his military accomplishments.
First, his military campaign in Gaul was impressive. He defeated an enemy that was known to be powerful and formidable. We can read of these battle in his own commentaries. In addition, he had a much smaller force.
Second, he was even able to defeat one of the greatest generals in Rome, who possessed many Roman veterans. Pompey during the time of Caesar was the man in Rome, and Caesar defeated him. This was no small feat.
Finally, his great generalship often times rest upon his speed and decisiveness. He was also known for his engineering accomplishments in warfare.
He helped transform Rome into an empire through his conquests. (Particularly his conquest of Gaul)