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.
Roman Republic
Since the foundation of Rome, its rulers had been monarchs, elected for life by the patrician noblemen who made up the Roman Senate. The last Roman king was Lucius Tarquinius Superbus ("Tarquin the Proud"). In the traditional histories, Tarquin was expelled in 509 because his son, Sextus Tarquinius, had raped the noblewoman Lucretia, who afterwards took her own life. Lucretia's father; her husband, Lucius Tarquinius Collatinus; and Tarquin's own nephew, Lucius Junius Brutus, mustered support from the Senate and army, and forced Tarquin into exile in Etruria.