When assessing Darius as a Persian King there are four main categories that we can assess him in:
Darius's most significant contribution to the Persian Empire was the refinement of administration. While Cyrus created an effective administrative basis as Jennifer Lawless said "Darius perfected it". The system of administration that Darius used was followed by all the Persian Kings until its collapse. The key to Cyrus's empire was that instead of conquering his neighbors and putting them into slavery, he allowed them to become a part of his empire, giving them everything they had before. This included the people's religious policies. The main difference was that a satrap replaced the highest level of government. This allowed for an extremely rapid expansion of the empire. Darius kept this characteristic of Cyrus's as well as most of his other policies. Darius refined the system by increasing the number of satrapies from approximately 20-30 so that no one satrapy could become too powerful.
Each satrap was also appointed various controllers and coordinators to minimize what power they could achieve. Coinage, weights, and measures were standardized. The Egyptian laws were codified. Darius created a standard taxation system for each of his satrapies, with the wealthier satrapies paying more that the not so wealthy. By making these seemingly simplistic changes to the administration system he set the standard for all the future rulers of the Empire as well as stabilizing the shaky reign of Cambyses. Darius is best known for his administrative genius and when he is compared to other Persian Kings he is unrivaled save Cyrus the creator, however as M.A. Dandamaev tells us unlike Cyrus, Darius' tactics were very low risk and were more focused on stabilizing the empire rather than expanding it. Upon examining all the above...