It all depends, and it differs from fighter to fighter. Losses and wins, and the details contained in them, determine a career and tells a storyline of that said career.

When Tyson lost to Buster Douglas, it wasn't an indicator that Tyson was a bad fighter, it was a showing that Tyson wasn't in his prime anymore.

Muhammad Ali's win over George Foreman is greater than any win on his record because of how much the odds were against Ali in that fight. His loss to Frazier is always a telling fight, because it proved that Ali had heart, and he had chin.

There are fighters that always come to fight, and no matter whether they win or they lose they put on a great effort. George Chuvalo is best remembered for his efforts against Frazier, Ali, and Foreman, and rightfully so. Sometimes, the challenge that a contender yields in a fight is the biggest concern whether than if they won the fight or not.

So, really, it is subjective.