Answer:
I would say that his main achievement was his ability to unite the whole Greece under his reign and authority after battle of Chaeronea.
Step-by-step explanation:
While he was a child, Philip was sent into Thebes as a part of treaty, which led him to be more and more acquainted with their culture, military tactics, governing system.
Therefore, when he started ruling in Macedonia he tried to use all of these features to strengthen his authority and later to spread it across whole Greece.