Haha, not a lot to add to Slim's post really, it all depends on the fighters and where they are in there career. In any case where you could make the arguement that a boxers best opponent beat him it is obviously going to hurt their standing, unless they are the perrenial contender type. For instance, Naseem Hamed never faced a better opponent than Barrera, and the manner of his loss in that one plus retiring afterwards makes it dead easy for anyone to assert that he was never as good as he was. Roy Jones scraping it so close to Tarver and the losing to him after so many years without a really live opponent(in hindsight) hurt his legacy the same way, etc etc.
Or take Shane Mosley, for instance. If he had just retired a few years ago he would forever be in the fantasy matchup debates with current welters like Floyd or Manny. Tonnes of people would probably still be talking about how he would've whipped them both. Now whether that were true or not it doesn't matter because and OLD MOsley has gone and embaressed himself against the pair of them.