I remember Floyd being rocked, but I'm not sure he ever had a official Kd in his entire career.
He took a (bogus) count vs Carlos Hernandez after he hurt his hand & turned away & got away with touching down after being Zabbed by Judah.
If anybody have such stats for Floyd, Sugar Ray, Hagler, Calsaghe,Pernell ect?
To the OP: quality of opposition matters in this regard. Calzaghe was only down versus Byron Mitchell, Hopkins & R. Jones Jr IIRC, but as his record has 35-something cab drivers on it, it means less than the bare stat shows. Definitely had a solid chin on him, though.
Hagler was only ever down officially once, when he touched down without a punch against Juan Domingo Roldan (ref blew the call).
V. Klit was never knocked down as a pro, but got put spark out as a kickboxer (though to be fair I believe that was from a kick, not a punch).
Marciano got decked by Jersey Joe Walcott (first fight) & Archie Moore. There may have been others (he tended to run face-first into counters squared up).
Whittaker suffered more than a knockdown or two; Buddy McGirt definitely put him over (left hook IIRC—what else was it gonna be from Buddy?!?!) in their first fight, I believe. He also got sat on his arse by Julio Cesar Vazquez, the Argentine light-middle, & by a couple of other dudes when he was at welter.
Sugar Ray Leonard got put down by (deep breath) Kevin Wosshisface (the guy he faced on his first comeback after Hearns nearly took his eye out? Howard? Kevin Howard?), then by Donny Lalonde, then twice by Hearns in the rematch, then twice by Norris & then I don't care what happened versus Camacho 'cos great fighters embarrassing themselves well into middle age shouldn't count, IMO.
Ray Robinson got decked a few times when he got older, too, but not many (despite the hundreds & hundreds of fights) until he was sickeningly old & I can't remember when or who off the top of my head.
Mike McCallum never got put over until he was like 40-something (once versus Jones Jr & once versus one of the Tiozzos, IIRC) & about twenty pounds over his best weight. He fought some savage hitters, too, & didn't blink in his prime.
All of these guys had fantastic chins.