#2 Conor McGregor vs. Khabib Nurmagomedov
The only faceoff on this list that Conor McGregor was not able to follow through with in the octagon. The bad blood between McGregor and Khabib Nurmagomedov was real, with McGregor insulting both Khabib's family and religion. The fact that Nurmagomedov reacted so little during the pre-fight press conference and interviews seemed to anger McGregor more and more.
This boiled over to the weigh-ins when the two men were faced against one another. Khabib took a fighting stance as is typical, but did not engage with McGregor in any way other than to lock eyes with him. McGregor, on the other hand, was chomping at the bit to get at Khabib.
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When Dana White allowed him within touching distance, McGregor slapped Khabib's hands down, screaming all manner of insults at him. It later became clear that Khabib had been holding in huge amounts of anger as he submitted McGregor and engaged in a post-fight brawl with the Irishman's team.
#1 Conor McGregor vs. Jose Aldo
The build-up to Conor McGregor's first UFC title shot truly cemented his status as an international celebrity. McGregor and the then-featherweight champion Jose Aldo went on an international press tour, with McGregor constantly hounding Aldo the whole time.
However, Aldo somewhat flipped the script when the two men faced off at the ceremonial weigh-ins. The Brazilian adopted McGregor's traditionally wide karate stance and put his hand in McGregor's face as the two men closed in.
This of course incensed McGregor, who attempted to front kick Aldo twice. The crowd bounced off the onstage conflict, with the whole stadium screaming its approval for the violence to come. When the fight did happen, it was over in a matter of moments, with the actual face-off possibly lasting longer than Aldo did before Conor McGregor left him face down in the octagon.