Which one of Justinian's accomplishments is the most impressive and why?
Of course, this is a matter of personal opinion. There is no way to objectively say which of Justinian’s accomplishments is most impressive. I would personally choose the creation of the Justinian Code as his most impressive accomplishment.
The reason I say this is because this was the accomplishment of Justinian that had the greatest impact in the long run. Justinian’s conquests were not sustained for very long. They were taken away from the Byzantines rather quickly. By contrast, the Code of Justinian had a long legacy. This was a codification of Roman law. The empire that Justinian ruled had access to huge quantities of Roman legal documents. Justinian had people go through all of the laws, legal commentaries, court decisions, and other documents and compile them into legal codes that could be used to govern the empire. This was a very important work. It was important enough that it continued as the basis for Byzantine law for more than 900 years. It also became the basis of the legal system of all of continental Europe.
To my mind, a legal code that endures for hundreds of years and becomes the basis for laws across a whole continent is a very impressive accomplishment.