Holy Roman Emperor
The Holy Roman Emperor was, in effect, a misnomer in spades. He was holy only because (in general) he took the title only after being crowned by the Pope in Rome. The Roman had little or nothing to do with Rome itself, and far more to do with the claim of the Popes to be empowered to pass on the potentia of the classic Roman emperors to a new monarch. And he was a king rather than an emperor.
The first post-Roman emperor so declared was Charlemagne or Charles the Great, who was crowned by Pope Leo III in 800CE as part of a deal whereby he agreed to champion and protect the Catholic Church. With a brief interregnum (891-898) whilst the Dukes of Spoleto tussled over the crown of Italy, the imperial crown passed to Charles' descendants until 924CE.
There then follows a break until 962CE, and the coronation of Otto I. After this, the Emperor was the king of, first, the East Franks, and, later, the Germans. An electoral system (see article on the Holy Roman Empire) provided kings/emperors until 1508, by which time the titles had passed firmly into the Hapsburg family, and lineal descent took over again. This was also the point at which emperors ceased to go to Rome, and technically became simple emperors-elect.
In practice the title of "Emperor" referred to the relationship with the Church :: to fulfil his feudal place, the title of "King" was of greater importance.