It's not like we have a great record of precisely what his accomplishments were, despite his reign probably being the best attested at least in terms of Persian sources.

Anyway, by far his most important accomplishment would be keeping the realms intact after assassinating Bardiya, usurping the throne and elevating his own clan to rulership, succesfully repressing the rebellions that by his own account broke out - and somehow convincing people of his bizarre cover story for assassinating Bardiya.

Second, he appears to have been responsible for some type of reform, generally said to be the creation of "Satrapies" by Herodotus' account. The appointment of governors was obviously not something new, so it has been suggested (by e.g. Briant) that what he created a robust and reliable system for raising revenue, by demanding a fixed amount of tribute based on the amount of arable land in each administrative region, in addition to taxes and tithes on production, trade and capital (such as livestock), otherwise remaining relatively hands-off in the administration of each satrapy. The main sources for this are Herodotus (who gives a tabulation of the amount of tribute demanded) and Pseudo-Aristotle's Oeconomicus (that details revenue raising in a "Satrapial" economy). In additon the Persepolis tablets give insight into the economics of Persia proper.

Third, he created a long-lived and powerful royal ideology, a theocratic institution centered on the absolute and divinely sanctioned authority and righteousness of the Great King, exemplified in the Behistun Inscription. It can hardly be overstated what an accomplishment it was to create an ideology and administration that would allow the incredibly vast Achaemenid realms to flourish for a century and a half after his death, in spite of rebellions, succession crises, and power struggles.

That I would say is a good summary of his accomplishments.