Pope Alexander III
Imagine dedicating your life to the service of the Catholic Church and your lord and savior Jesus Christ, working your way up through the ranks with your eye on the top job--though you'd never admit to it, cuz, you know, ambition's kind of a bad look for a pope hopeful, and just when you reach the mountain top, when the papal conclave tabs you as the capo di tutti capi, some other clown is elected antipope. Sucks, right? You know what sucks even worse? Having your papacy cramped by four--four!--antipopes. That's exactly what happened to Rolando Bandinelli, aka Pope Alexander III.
Bandinelli served under Popes Eugenius III and Adrian IV, eventually becoming chief papal negotiator with Emperor Frederick I, aka Barbarossa--the original Red Beard. Baldinelli grew concerned that the Emperor was growing too powerful, and one day in 1157 let it slip that he believed the empire was a "benifice" of the papacy, which is to say the empire and the emperor's power existed by the grace of Rome, and so were subordinate to the church. Barbarossa was pissed.
On September 7, 1159, following the death of Pope Adrian IV, Bandinelli was elected by the majority of College of Cardinals and took the name Pope Alexander III. But enough cardinals cast their ballots for Octaviano Monticelli that Montecelli, with the support of the still steaming mad Babrarossa, took the mantel of Antipope Victor IV, thus kicking off a power struggle pitting Alexander III against Victor IV and Barbarossa. Alexander III's first salvo was to excommunicate Barbarossa and Victor IV, but there was enough heat that Alexander III had to bolt from Rome, running off to France where he stayed from 1162 to 1165, as he built support in France, England and other Western European nations.
Victor IV passed away in 1164, but Barbarossa was still angry enough to support the election of Antipope Paschal III, who died in 1168, and you'd think, finally, our man Alexander III could enjoy the job, but no, and so we got Antipope Callixtus III, who "reigned" until his death in 1178.
Spoiler alert: 1178 saw the election of Antipope Innocent III, who had the backing of Victor IV's family. Just ninety-five days later, however, Pope Alexander had officially had enough of this bullshit, and, having amassed enough juice, had Innocent III shipped off to La Trinità della Cava, where he lived out the rest of his days.
In the midst of all this nonsense, Alexander III and Barbarossa were going hammer and tong for supremacy--with the latter's missus getting embroiled in the whole megillah, before the two men finally signed made nice and signed Treaty of Venice.
Pope Alexander lived for another year and half after disposing of Innocent III, dying on August 30, 1181, a week short of his twenty-second anniversary in office. His reign of twenty-one years, eleven months and twenty-four days stands as the seventh-longest papacy ever, not bad for a guy who had to contend with more antipopes than any pope before or since.