Roman emperor
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the West after his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
Constantine VI
Constantine VI was the final ruler to be universally recognized as Roman Emperor, being recognized as such by both the Empire which he ruled in the east, the papacy and the Western European powers over which the pope held suzerainty. With his mother becoming Empress regnant upon his deposition, the papacy crowned Charlemagne as a new Emperor in Western Europe, asserting that a woman could not be Empress in her own right. This laid the foundations of a new polity, independent of the East, that would evolve into the "Holy Roman Empire".