I feel it was Khabib v McGregor, does any other MMA match-up come close? Will those PPV numbers ever be eclipsed?
Are you referring only to the number of pay-per-view buys or are we also taking into account the significance of the fight to the legacy of the sport? As far as potential fights that could surpass Khabib/McGregor numbers, I think a fight between GSP vs Khabib or GSP vs McGregor could theoretically do more buys. Especially the latter.