The Last Sole Ruler of the Roman Empire

Theodosius was an enormously influential ruler, and though I don't think he cracks into that upper tier of truly great emperors, there is no getting around the fact that the history of the world runs right through his reign. His staunch Catholicism and eventual staunch anti-paganism spelled the end of the old religions of antiquity. Monotheism was here to stay. Christianity was now the religion of the Empire.