#2. Jairzinho Rozenstruik vs. Augusto Sakai - UFC Fight Night 189

The UFC and their love of heavyweight action was on full display in this one. Both Jairzinho Rozenstruik and Augusto Sakai had once been deemed as potential title challengers. However, their title aspirations had been dimmed significantly prior to this fight leaving a main event that was far from electric.
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Rozenstruik seemed destined for big things when he knocked out Alistair Overeem to maintain his undefeated record. He then lost to both Francis Ngannou and Ciryl Gane in the space of three fights, which significantly cooled the hype surrounding him. Sakai had also lost a lot of steam prior to competing in this one.
After a great start to his career in the octagon, Sakai suffered a TKO loss to an aging Alistair Overeem that showcased a number of holes in his game. These losses took all the shine away from this main event that ultimately produced a TKO win for Rozenstruik.
#1. Aspen Ladd vs. Norma Dumont - UFC Fight Night 195

The planned main event for this card was Ketlen Viera vs. Meisha Tate. While that wasn't exactly the highest profile main event, the bout had to be re-scheduled and was replaced by Holly Holm vs. Norma Dumont. When Holly Holm pulled out injured, Aspen Ladd vs. Norma Dumont ended up in the main event spot.
While there were several late cancelations that led to this main event, it's tough to deny it is the least star-studded in recent memory, potentially of all-time. Neither fighter had made much of a name for themselves in their UFC careers and neither have won since competing in this bout.
On top of this, the fight took place in the featherweight division where there are only a handful of fighters and no real names of note outside of Amanda Nunes. This fight had no place being in the main event and you can only hope the UFC will be able to pull together a more enticing headliner if put in this position again.