Daniel served several kings during his time in the Bible, primarily during the Babylonian and Persian empires. These include:
* Nebuchadnezzar II: The king of Babylon who conquered Jerusalem and took Daniel and other Jews captive. Daniel served him and interpreted his dreams.
* Belshazzar: Nebuchadnezzar's successor (or possibly grandson) in Babylon. Daniel interpreted the writing on the wall during Belshazzar's feast.
* Darius the Mede: A king who ruled Babylon after the fall of the Babylonian Empire to the Medo-Persian Empire.
* Cyrus the Great: The Persian king who conquered Babylon and allowed the Jewish exiles to return to Jerusalem. Daniel served him as well.
So, Daniel served under Nebuchadnezzar II, Belshazzar, Darius the Mede, and Cyrus the Great.