Wikipedia says that King Juan Carlos I of Spain not only claims the title King of Jerusalem (although several others, including Victor Emmanuel of Italy and Otto von Hapsburg could also make the same claim), but could claim the title Emperor of the Romans, the last Paleologos pretender having given the title to Ferdinand II.
Of course, the Ottoman Sultan Mehmed II claimed the title Ceasar of Rome when he conquered Constantinople. Given that the list of recognized monarchs of any given kingdom has as many successions by force as by birth, I suppose we should arrange a cage match between Juan Carlos and His Imperial Highness Prince Sultan ErtuÄŸrul Osman V, the current Ottoman pretender. Juan Carlos has the advantage of actually ruling a kingdom, although he voluntary gave up most of his power when he gained the throne.