Are Hagler's title challengers underrated?

Or even gone 15 rounds with Hagler ?
I think he was at least as good against Hagler as he was against Barkley, and I could only give him 4 to 5 rounds at the most. Hagler just didn't give him the same openings.

Well, yes, this has been discussed. For whatever reason he didn't. In the grand scheme of things it's not so bad. Some great fighters did such things all the time, and get a pass.
But Hagler's held to a different standard because it wasn't expected of his style.

I think almost every great champion has a challenger who takes them the 15 rounds distance. Often a fighter no one expected to do so well.

Im not sure what you think of Duran the middleweight. Is he "just another contender" or something more or something less ? It doesn't really matter.

All I can say is that Hagler defended the title with a clear decision win, one that didn't warrant a rematch, and a performance that would suggest a rematch would probably result in an even more comprehensive win for the champion.
Then on the the next challenger.

Well, I'm all for using a cut-off point in every great fighter's career to ignore all the sad a pathetic late career defeats when they were clearly a shell.
But I say if you're going to include a result to add to someone's legacy/ATG rating, you shouldn't ignore the results that come in that same period.

The case of Ezzard Charles, for example, I would totally ignore everything that happened after the first Marciano fight.

Archie Moore is an opposite case. He was actually pretty good when he was past prime, all the way to the end. He was KO'd by Cassius Clay, who was already a very good fighter. Moore had many good wins in his last years of boxing.

:good