Augustus' Greatest Achievement would be : The Pax Romana , which mean Roman Peace
Before augustus' Reign, The Roman empire was under a serious Crisis. A lot of Corruption happen in the country, incompetent government officials, a lot of wars between roman states, etc. During his reign, Augustus managed to solve all of this problem, creating a Glorified prime era for Rome. This is why he's considered as one of the best Roman emperor in the history