I am not sure that I am very impressed by any of Justinian’s accomplishments other than the creation of the Code of Justinian. However, there are other things that some people could find impressive.
To me, the most impressive of his accomplishments was the compilation of the Code of Justinian. It is impressive that he cared enough about the law to have people go through all of the Roman legal documents that the Byzantines had. It is impressive that he had them create an organized set of laws that formed the basis of Byzantine law until the empire collapsed some 900 years later. It is impressive that the code was useful enough that it became the foundation for law in all of Europe.
However, there are other things that some people would call impressive accomplishments. One of these is military. Justinian regained a fair amount of territory that had been part of the Western Roman Empire. He conquered Italy and parts of Spain and North Africa. Other people might be impressed by the building program that Justinian instituted in Constantinople. They might be particularly impressed by the creation of the beautiful Hagia Sophia church. I would argue that these actions hurt the empire by overextending it and draining its treasuries, but some would call them impressive.