Final answer:
Cyrus the Great was the founder and first king of the Achaemenid Dynasty of the early Persian Empire, starting around 550 BCE. His successors, including Cambyses II and Darius I, continued to expand and reform the empire.
Step-by-step explanation:
The founder and first king of the Achaemenid Dynasty of the early Persian Empire was Cyrus the Great. His reign began around 550 BCE when he succeeded in defeating the Medes, thus establishing the expansive Achaemenid Empire that spanned from the Balkans to the Indus Valley. Further expansion occurred under his son, Cambyses II, who conquered Egypt, and then under Darius I, who not only increased the empire's size but also implemented significant administrative reforms such as dividing the empire into satrapies.