The one answer that absolutely no one can quibble with on any moral grounds: Luis Santana of Terry Norris fame......
Many things mentioned here are not quitting lol, try fighting with a torn shoulder and see how long your career. I’d say Duran, depending on the myth surrounding it he either stopped because he had a stomachache (lmao) or was simply pissed off at Leonard. Both are equally bad.
Alexander Dimitrenko against Tony Yoka / Andy Ruiz Jr. Michael Wallisch against Christian Hammer / Tony Yoka / Joe Joyce. Fabrice Bénichou (because he want to give up boxing so he make a knee even if he can still fight and the referee stopped the fight) against Spencer Oliver.
Hence why it means nothing in the grand scheme of his career. If he had managed to go the distance and win - how could anybody possibly venture Greb as his rival for GOAT? TBF, unless miraculous film shows up of him looking everything the legend suggests, I don't see how anybody puts Greb over Robbi anyway - the film means a lot IMO.
It would certainly have been impressive, but I think Duran vs Barkley is the most impressive single win in boxing history (that I can think of). SRR certainly looks the best on film to my eye, but honestly I just refuse to draw a firm conclusion one way or the other about Greb.