A difficult question and many candidates. But here is my opinion. Thomas Hearns Welterweight (147 lbs).
I'm surprised no one has mentioned Mike Weaver or Cleveland Williams yet. For me by far the Boxers with the best physics. Also an interesting story when williams was shot by a cop with a 357 Magnum. Quote: "I've never seen a man with such a constitution," Dr. D.L. Bricker said after the surgery. "He lost enough blood to kill a half-dozen men." "Doctors told me they had never seen such muscles on a human being," said Hugh Benbow. "Anyone else, they said, would have been killed by the shot."
As far as being ripped, Dick Tiger and Lou Brouillard haven't been mentioned yet. Gene Fullmer was pretty well put together and in shape. Basilio too. Milo Savage was another. As far as real old-timers go, Bob Moha was a real hunk. I've always thought Chicago's Bobby Boyd was well proportioned for a fighter who had to make weight: he had a big upper body for a middleweight and slender legs. Not so much ripped, but he always looked bigger to me than other boxers in his weight class. He used his upper-body weight to advantage too.
Cruiserweight Evander Holyfield at 6’1 and a finely chiselled 190lbs seemed like the perfect fighting machine with a physique to match.
Off the top of my head Norton, Ibeabuchi, Tyson, Wlad, Holyfield, Haye, Bradley, Joshua, Hagler, Andy Ruiz
Is there an optimal physique type for boxing? For instance, I remember Jim Lampley saying in regards to a somewhat flabby boxer that “that extra fat helps him absorb bodypunches”. Is there any truth to that? Muhammad Ali was the best I’ve ever seen at absorbing body shots. He showed this in FOTC, Zaire, and especially Manila. He wasn’t quite as lean in these years, he had a slight bit of fat. In his younger years when he was leaner, I saw him doubled over once briefly from a body shot (Mildenberg). Still, this was just one incident and he recovered pretty fast. James Toney also had ribs of steel. Guys used to pound away at his midsection. He went from being slightly pear shaped early on to being, in the words of Jim Lampley, “a fat tub of goo”. These are just 2 examples though, and it might be a coincidence. But I was wondering if there’s any ideal physique for boxing.
Andy Ruiz Chris Arreola Butterbean Gabe Brown Dustin Nichols Those guys have bodies sculpted like Hercules.
If there's any benefit to being a bit tubby, it's only there at HW. At anything else, you'd prefer that extra weight be used for reach, height, and muscle. As to being tubby, Foreman was hurtable when young and lean, but when old and fat, was a tank. Did anyone even manage to rock him in his second career? He'd get banged around plenty, but never seemed hurt.
The fact that people are saying Ali had the best ever physique is evidence that this board is more dedicated to endless worship of the great fighters of old than actual analysis. Was his physique great during his first reign? Yes. A better physique than Beterbiev, "Big Worm" Nichols and Tanaka? Come on. Tanaka at 125/5'4 would be 235 pounds at Ali's height, and look at how little fat he has compared to Ali.