For me, it’s definitely Lennox Lewis. While his career as a whole doesn't impress me quite as much, his comeback after the loss to Rahman certainly does. At the start of the year, he gets brutally knocked out, only to deliver the best performance of his career in the rematch. Regardless of Rahman's caliber, it was one of the best knockouts I’ve ever seen, and Lennox demonstrated both class and a champion's mindset. Roy Jones and Montell Griffin are another such example, though there are plenty of others. Did anyone expect Roberto Duran to still be at the top after the famous "No Más" and that devastating KO by "Hitman" Hearns?
Hey cheers Fergy. Using the Ali loss is interesting. It was devastating, he went into a mental spin. He did however beat Lyle and Frazier back to back straight after. They were top 5 heavyweights. That's a good start to coming back even if it derailed later.
Another one that spring to mind is Nigel Benn. Caned by Watson but came back to reign well at super middle
Robinson came back and beat Castellani, Olson, Basilio, Fullmer, Moyer and Dupas. Foreman comes back to be HW champ at 45.
After getting KO'd in the 4th by Spinks things weren't looking good for Marvin Johnson, who had been in two wars with Saad, a tough fight against Galindez and fought on through terrible punishment against EMM. Despite looking like a soon to be retired fighter with his best behind him. He went on a successful 14 fight run before capturing the title for an unprecedented 3rd time in his standard thrilling fight with his future conqueror, Leslie Stewart. Even in his final fight, after getting knocked down twice in the first round and in trouble for the other 7, he remained on his feet and retired in his corner, on his stool. Still keeping a bit of dignity by knowing that enough was enough.