Couture's win was a big upset, if not at the very least a sizable upset.
Cormier vs Stipe Miocic was a close fight.
Tim was hurt going into that fight, if they had insurances back then like they do now where everyone pulls out with smallest injuries, I'm fairly sure that fight never happened cuz Tim got a surgery on his back afterwards for that injury AFTER that fight. Imagine fighting a world class wrestler with a back injury severe enough for a surgery? That's what Tim did.
Tim was really good though unpopular. But very unlucky. I don't think Tim loses to Couture healthy. If he didn't lose to Couture, he may never ended up fighting Mercer because even if with loss to Nog, he would've still been a former UFC hw champ with FOUR defenses. Miocic also has four total defenses.
Unfortunate.
This is a very tough call. Much bigger upset is Couture, but that win over Miocic really changed the narrative on Cormier. Without Jone's self destruction, he would have been a champ at neither hw or lhw as well but that was a huge win for Cormier, making him a double champ.
At the end of the day, given the forum reactioin, much bigger win is Couture over Sylvia.