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.
Holy Roman Empire
After being elected, the King of the Romans could theoretically claim the title of "Emperor" only after being crowned by the Pope. In many cases, this took several years while the King was held up by other tasks: frequently he first had to resolve conflicts in rebellious northern Italy or was quarreling with the Pope himself. Later Emperors dispensed with the papal coronation altogether, being content with the styling "Emperor-Elect": the last Emperor to be crowned by the Pope was Charles V in 1530.