What is statistically the worst defeat a top fighter suffered?

Billy Costello getting ko'd by Lonnie Smith. Costello was undefeated, Smith wasn't a bad fighter, but had a fairly low ko percentage, and yet floored Costello numerous times. Smith went on to lose the title in his first defense.

 

Didn’t a Journeyman beat Kid Galvin? I always ask this in threads and can’t figure out who it is on BOXREC.

 

shavers got bombed out by Brian Yates. wasn't yates like 5-82?

 

Matty Mattews losing a NWS decision to Joe Grimm.

 

Unless I've misread the hypothetical frame wrong - I would say Ali losing to Leon Spinks fits the bill.

Then there is also Evander Holyfield KO'd in 1 round by Victor Belfort.

 

Oooh, that's a nasty one. That gave Mr Grim a long, winning streak of 2 fights - appears to be a PB for his career. LOL.

 

Roberto Duran losing to a very good friend of mine…Kirkland Laing (RIP). Kirk was a very talented boxer, reflexes, speed, movement. He just had other technical faults…his tendency to leave himself open at times with his hands down style didn’t help!… and he was also fighting inner demons all his life…which unfortunately neutralised his natural talent.
But against Duran that night, Kirk’s talent prevailed against an ATG, and he deservedly outpointed the great Duran. I thought this would kickstart Kirk’s career, but it wasn’t to be.

 

Ali v Spinks was my first thought.

 

Just pure rec ranging and a very quick scan at that but I see The "Keed" dropped two 10 UDs to a Doug Ratford in '47 and '48.

Ratford's record as at the time of each bout and his overall record didn't look too impressive. Is he perhaps the guy you're thinking of?

Amazing to look up the old timer's record and see the amount of fights they had - a completely different breed back then.

 

Max Baer losing to 50-25-7 Braddock
Jimmy Carter losing to garbage collector Johnny Cunningham19-31-4 .Carter was mobbed up so draw your own conclusions
Roland Lastarza losing to 179lbs Rocky Jones 14-9-2

 

Thank you Delboy, that’s nice of you. Yes I have great memories of Kirk, not least because he took me and my pals to his boxing gym to train and get fit.
He was a few years older than us and he kept us out of mischief by getting us involved in boxing training, sparring and just generally getting us motivated about something other than just being tearaways. We stayed pals for a long time and I’ll always be grateful to him

 

Quite a few inexplicable results in the 40's and 50's. Many courtesy of Mr Gray.

 

Was light heavyweight champ Michael Spinks outpointing the unbeaten 48-0-0 a great upset? I believe not only was the defeat a bit of a surprise but the fact that Holmes was about to equal Marciano’s record added more flavour to it.