Stefan Uroš I of Serbia - often underappreciated and left in the shadow of both his grandfather and father and of his descendants. Similar to how Aleksandar Karađorđević is also called "King Unifier", I think Stefan Uroš I should have the same nickname.
Cyrus the Great by the usual standards of greatness. Meaning that we judge each historical figure in accordance with his own time, and no prior monarch to Cyrus was remotely close the the grandeur of his accomplishments. He is generally described as a benevolent rulers who freed his people from the rule of a tyrant.
Another good option is Chandragupta Maurya. Self made man who overthrew a powerful tyrant and created the most powerful empire of his time.
Augustus, Trajan, Constantine of the unified Roman Empire.
Justinian I of the Eastern Romans.
Greatest medieval European king - Charlemagne.
Greatest early modern European ruler - Frederick the Great, Peter the Great, Pitt the Younger, Oliver Cromwell, Charles V of Spain, Suleiman.