Leonard after Duran? (depending on how you interpret "better fighter") I'm skeptical about a lot of the examples listed though on "correlation isn't causation" grounds. Having greater career accomplishments after losing to someone doesn't mean that the loss actually made you a better fighter.
I read somewhere that jack Johnson got knocked out by Joe choyinski and then the two trained together in the joint That was before Johnson was a champ
I read somewhere that I mentioned this as the big example missing from the thread. Oh, that was on the last page.
After reading the question posed by this thread and giving it some serious thought, I am not sure the loss itself ever helps. A fighter learns as he goes along. He would learn just as much if he won the fight instead of losing. It isn't the loss, it's the experience that counts.
Hearns learnt to clinch after the SRL loss and ended up imo at his perfect weight in 154. Hagler came back a lot better after an early loss or two. Mike Weaver found a bit of confidence after Holmes. Hopkins improved a lot after Jones, how much that actual fight helped might be minimal tho.
As others have said, it's a bit of a grey area to gauge, because the fact that a fight was a loss doesn't necessarily mean that a fighter learned any more from it than they would do a win, or that they wouldn't have subsequently had the same success as they did. But if we do accept those premises, then what about Tunney against Greb? Tunney got an absolute shellacking in their first fight - Grantland Rice wrote that Tunney was, 'literally wading in his own blood', while Damon Runyon said afterwards that referee Kid McParland was covered in so much of Tunney's blood that he looked like, 'someone who'd been painting his house red while under the influence of hard cider'. According to Tunney's biography by Jack Cavanaugh, this fight (along with his increasing hand injury issues) convinced Gene that he needed to adapt his game and become more of an upright stylist prepared to go the full distance more often. The defeat also prompted him to seek the advice of Benny Leonard, who suggested to him that in order to get the better of Greb if / when they fought again, he'd need to develop a better body attack and stop head hunting against Greb, which hadn't been effective. Tunney duly did improve his body punching and utilised it effectively in later bouts with Greb, and carried this improvement up to Heavyweight. Remember, in his second fight against Dempsey, Tunney hit Dempsey with a straight punch to the chest, of which Jack said: "It was without doubt the hardest blow I've ever taken in my life. It was not a question of thinking I was going to be knocked out - I thought I was going to die."
I'm not sure it was the loss itself that made Tunney a better fighter or the lessons he learned during the course of the fight. Some people said that Joe Louis benefited from the loss to Schmeling. They said that he was getting over-confident and lax with his training, and that the beating he endured put him back on the path to success. I'd argue that the terrible beating he received in that fight was likely the primary reason for the findings by doctors in the 1940s that the difference in his reaction time between his right and left hands showed brain damage. Seems to me that the damage done by the beating he received outweighed any motivational benefit. In Tunney's case, I think he was a motivated individual, so much so that had he not lost the first Greb fight, he still would have eventually recognized his deficiencies and found a way to achieve his goals. I realize I'm boxing myself into a corner here and that there may be a few cases where a loss itself motivated a fighter to improve in a way that they would otherwise not have done. Tunney may well be one of them. I think, however, that boxing matches where a beating benefits a fighter are few and far between, and that the concept is over-rated by many boxing fans.