Wallace (Bud) Smith had an uneven career - but what do you expect when you're fighting the best out there - before receiving his title shot against the much-feared Jimmy Carter. But beat Carter to win the title. Moreover, he repeated the win in his first defense. But that was it. Amazingly, he lost his next 11 fights before calling it a day.
Ezzard Charles. His decline had already started when he got KO'd by Walcott, but he was still winning the vast majority of his bouts and only lost to the top names. But he was dropping decisions and was getting stopped against fighters he would have decapitated in his prime after his 2 fights with Marciano, and was 3-7 in his last 10 fights.
Elisha Obed. Seeing him as champ in '76 verses getting ragdolled by Czyz in 1980, you would never have recognized him as the same person.
Thad Spencer, heavyweight. Was upset by Jerry Quarry in the WBA tourney, where Quarry seemed to take the boxing life out of the guy with great counter punching. He never won another fight.
Don Curry was never the same after Honeyghan. Managed to win another title but just never had that something special he had as a welter. So sad losing to Jacquot and the beatings he took against Norris, Nunn and Linton were painful to watch. Honeyghan after the Starling loss. He started to lose it in the 1st Vaca fight but cameback so well but Marlon hammered him. The Breland fight has to be one of the saddest fights I´ve seen a former champion take part in.
Livingstone Bramble he was considered a top 5 P4P fighter after beating Mancini, Crawley. Then he got destroyed by Rosario and basically turned into a journeyman after that loss losing over 20 fights.
Would add Bowe. He was considered the top guy together with Tyson when he stepped into the ring with Golota. After those two fights he was finished.
There's also Terrell from that same time. Had quite a nice run going, a 5-6 year win streak, when he faced Ali. Never had a good win afterwards. Was incidentally Spencer's last good win, I think.