Does defeaating a fighter necessarily qualify him as better? Even when both fighters are in their prime?
It's different if they ended up winning the rematch. Some would say that Morales is better than both Barrera and Pacquiao. There's certainly and argument there. Williams vs. Margarito is an excellent pick. Most people would have Smokin Joe rated higher than Foreman.
Probably. Like for example Hopkins/Taylor. You can make the argument that Hopkins won, but even if you didn't, does anybody really think Taylor's a "better" fighter than Hopkins? Of course not, but Taylor got the W. There are many factors why a fighter beats another fighter, that doesn't prove that the fighter is actually better. It can include bad judging (too many examples), bad fight plan (like Hopkins starting too late to get going against Taylor), or a bad styles matchup. Although that being said, I think these are in the minority. Most of the time the fighter who wins, its because he's actually better.
If you eliminate controversial decisions, losses that were avenged, and take fighters close to their prime ages and weights, it's harder to come up with examples. Junior Jones-Marco Antonio Barrera seems like it would work, though there were those who gave MAB the nod in the rematch. Kostya Tszyu's all-time status is higher than Vince Phillips', even though Phillips knocked him out.
I agree that its close to come up with a bunch of examples (Jones/Barrera and Tszyu/Phillips are good ones). Maybe Mosley/Forrest? Both were in their prime when they fought, and I don't think Forrest is a better fighter than Mosley, but maybe there are some that disagree.
I can maybe see the argument for Morales and Barrera. But over Pacquiao? No way. agREED with X and Sly on Frazier/Foreman. No more needs to be said on that.
I'm kind of surprised that nobody's yet mentioned Mosley-DLH. Historically, I'd have to believe that Oscar will rate higher than Shane. Barkley-Hearns was already mentioned but is probably the very best example. Not sure if Charles Murray ever registered on anyone's radar, but I'd have to believe he was more highly regarded than Ray Oliveira.