Which of Justinian's accomplishments had the longest lasting impact on western europe

It would Justinian's Code. The code has become the basis of which many have built their own law codes on around the world.

Justinian's Code - - set of laws and punishments coinciding with the laws.

Justinian created a code of laws to bring order to the Byzantine Empire. The laws created by Justinian created a set of universal laws which acted as national laws which had ultimate power over the people in Justinian's empire. Each of the laws corresponded with a punishment which also set up a comprehensive court system along with the laws. This legal system was copied and replicated in many European governments as well as the US system.