What was Julius Caesar's MAIN achievement in Rome?
This question calls for a lot of subjective reasoning so that one person's answer could never be said to be definitive. It is like asking "What was Abraham Lincoln's greatest accomplishment?" No two historians would answer that question the same way.
Nevertheless, you question deserves an answer and so I will offer you my opinion.
If you look to any result of a Google search for an answer to your question you would likely see many of the same things repeated.
He was a military leader who, according to many, made the Roman army the most feared of its time.
He was a great orator, impressing people so much that he was named a priest of Jupiter.
Many other accomplishments are attributed to him as well.
But, perhaps, his most important accomplishment was the centralization of the Roman Republic. Never before had the government of an empire as large as Rome been so representatively ordered. With its Senators there in Rome, accessible to Caesar and by Caesar to address the concerns in any part of the empire at any given time, it caused the government to run more smoothly than any other before it.
Many aspects of it being mirrored by others that followed, including Britain, and to an extent America.