#2 Tony Ferguson vs Khabib Nurmagomedov - UFC 223

Which instance of Tony Ferguson vs Khabib Nurmagomedov being canceled do we choose for the purposes of this article? For both men, a potential meeting would've been the biggest fight of their respective careers.
For Ferguson, a man who was on a massive win streak and desperately needed a shot at the undisputed title, Khabib was the only man standing in his way. And for 'The Eagle', who had ran through his competition with relative ease before staring in the face of a championship fight, Ferguson was his toughest match-up both psychologically and athletically at the time.
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But ahead of UFC 223, 'El Cucuy' tripped over some wiring and blew out his knee, leading to Khabib facing Al Iaquinta for the title. And once the Dagestani laid hands on the gold, even an army wouldn't have been able to pry the strap away from him.
Ferguson returned after a brief layoff and took part in another interim lightweight title fight, but he was never the same after the injury. The crowning moment of his brilliant UFC run never came, a moment that would've etched his name in history had he beaten Khabib.
#1 Jon Jones vs Daniel Cormier - UFC 200

Another match-up that was cursed in a way, Jon Jones vs Daniel Cormier had pull-outs and controversies from both sides.
Jones became the first to withdraw due to a leg injury suffered in a press-conference brawl, but the two fighters eventually met at UFC 182. Jones won the fight handily, but became involved in a series of controversies outside the octagon. Once a rematch was scheduled, it was DC's turn to pull out with injury.
Jones vs Cormier 2 was scheduled for UFC 200, but 'Bones' tested positive for banned substances, leading to another suspension. This might've been the most telling episode of the whole saga, the incident that makes us claim Jones didn't turn up to the most important fight in his life at the time.
Jones had beaten Cormier once already, but DC had brushed aside every other opponent he had faced. A rematch was inevitable, and Jones had already worsened his case by somehow involving himself in almost every controversy imaginable.
Finally, after spending significant time on the sidelines, he had the chance to redeem himself with another win over Cormier in a historic fight for the UFC. But Jones didn't show up. And even when he eventually did, the UFC 214 fight was overturned to a no-contest because of another controversy.
Jones is undoubtedly one of the greatest fighters in MMA history, but the events that transpired in the lead-up to UFC 200 weren't flattering, especially given the magnitude of the event.