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.
List of Roman and Byzantine Empresses
The Western Roman Empire met its end in 476 and the Eastern Roman Empire in 1453. Although others continued to claim similar titles after the Fall of Constantinople in 1453 — e.g. Holy Roman Empresses (as heirs of the Western Empire) or Russian Tsaritsas and Empresses (as the Empresses of the Third Rome) — the last reigning Empress consort of the Eastern Roman Empire of Constantinople was Maria of Trebizond. The last Palaiologan pretender, Andreas Palaiologos, sold his right to the imperial succession to Charles VIII of France, but he also willed the imperial titles to Ferdinand II of Aragon and Isabella I of Castille, and so in a sense either the French queens or the Spanish queens have been the titular Empresses of the Eastern Roman Empire since the 15th century. Another Palaiologian, Manuel Palaiologos, sold his right of succession to Ottoman Sultan Bayazid II (the Ottoman sultans already claim to be the Kaizer-i Rum or Roman emperors); but since there is no such thing as a sultaness because the Ottomans practiced polygamy, there are no Ottoman consorts. Other possible pretenders may be the former Queens of Greece because the Greek monarchy was mainly created in 1832 to be the successor of the Byzantine Empire. The former Queens of Italy could be another claimant since their husband's were one of the only European monarchs to effectively hold the city of Rome, the seat of the Roman Empire since its beginning. 
Neither the Empresses of Russia, the Queens of France, the Queens of Spain, the Queens of Italy or the Queens of the Hellenes claimed any sort of Roman titles and the claimants that clearly made the most point by using the word "Roman" in their title, the Holy Roman Empresses and the Queens of the Romans, ceased their claim upon the dissolution the empire in 1806. Currently the consorts of five of these states are pretenders in their own countries, themselves, and the current Queen of Spain claims no Roman titles. The status of the current pretenders to the Byzantine successor states of Trebizond, Epirus, and Nicaea are unclear much less their spouses; the despots of Morea became the Byzantine emperors (in exile) in 1453.