This was one of the first names that came to my mind when I saw this thread topic. Some other examples: Lew Jenkins - Famously burned the candle at both ends. Had a few good performances after winning the LW title, but then went on a steep decline that continued well after he lost the title, & never again was a world class fighter. Jeff Chandler - Showed up to a title defense vs. Richie Sandoval looking utterly devoid of any spirit, got stopped for the 1st time in his career, & never fought again. Nino Benvenuti - There were red flags even before he lost his title when he struggled in his non-title fights, including a huge upset stoppage loss to Tom Bethea. After losing his title to Monzon, he got his a$$ kicked in what was supposed to be a "warm-up" fight, then got stopped easily by Monzon in a rematch & never fought again. Frank Klaus - Restored order to the MW division after Ketchel's untimely death by beating all other claimants to the title. Having finally established himself as the undisputed champ, he was then shockingly stopped twice by George Chip (the only 2 stoppage losses of his career) & then retired. Freddie Steele - Got shockingly KO'd in 1 round in a title defense & then announced his retirement the following day. His sudden decline has been attributed to the sudden death of his longtime manager/mentor the previous year & a recurring chest/rib injury. Steele had a heckuva resume & the sudden/anticlimactic end to his career is probably the biggest reason he isn't more widely known/heralded when great MWs are discussed today. Virgil Akins - Had an impressive run of wins that culminated w/ winning a tournament to determine a new WW champ, but lost the title in his 1st defense & then spent the remainder of his career as a trial horse for up-and-coming WWs. Akins is widely rumored to have been mob-controlled & many people speculate that he "lost" his title on orders, but I've always speculated that he may have simply been burned out from all the wars he had before & even after winning the title. Don Jordan - Akins' successor as champ, who declined almost as swiftly as his predecessor after winning the title. Sadly, Charles would ultimately pass from Lou Gehrig's disease, which is believed to have already emerged by the time of some of his late career losses.
Olivares' decline reminds me of Saad Muhammad's - they both showed increasing wear & tear in their last 2-3 title defenses until showing up to one utterly weight-drained & getting dominated. Unlike Saad, Olivares did have a commendable "2nd career" up at FW, but never regained the same dominant form he once showed at 118.
Jeff Fenech was basically finished after the first Nelson “draw”. Jeffs punch resistance totally left him and a once solid chin became suspect as hell. And even though never a single shot power puncher Jeff’s punches looked almost feeble in the Nelson 2 and Calvin Grove fights. He’d basically lost the fire and Fenech was a guy who was at his best when he had that burning desire.
John Mugabi, may not have been the absolute worst fall -off, But after his war with Hagler, he was clearly not the same. Most of the Philly fighters seemed to just fall off a cliff at unexpected points in their careers... Probably because of those legendary gym wars so many different sources wrote about from the mid 40's to the early 90's.
Freddie Steele went 120-2-11 until Fred Apostoli broke his breastbone aged just 24. He was effectively finished then and ended his career 3-3.
Johnny Bumphus went down the tubes quick too. My lingering memory is of him stumbling around the ring helplessly against Honeyghan.
Roy Jones after moving back down from heavy. He clung on just enough to win Tarver 1 but was pretty much completely finished after that.
"Irish" Bob Murphy received a body beating from Joey Maxim in his lone title shot, & his career quickly plummeted downhill afterward.
Jermain Taylor Ray Robinson after Basilio Lupe Pintor after Meza Frankie Randall Leon Spinks Jimmy Young Quick Tillis