Andy Ruiz
Ruiz stunned the world in 2019 when he became the first Mexican heavyweight champion ever. Anthony Joshua chose him as a last-minute replacement after Jarrell “Big Baby” Miller failed a drug test. The pair fought in Madison Square Garden where Ruiz shocked everybody with his incredible knockout win (via The Sun).

This ranked up there with Buster Douglas’s KO of Mike Tyson as one of the biggest shocks in boxing history. Joshua activated his rematch clause and Ruiz didn’t take the rematch seriously. His lack of discipline cost him dearly before the second fight. Joshua outpointed him to recapture his titles as Ruiz blew the biggest opportunity of his life.