I don’t think we can assume that every boxer who came along after the invention/discovery of steroids used them, so it’s kind of a trick question. On the other hand, there’s a point in boxing history where weight training became acceptable and even common whereas before the thought was it just built muscle mass that led to a fighter being slower, maybe going up in weight class past his most effective weight, and less flexible. But not everyone who embraced weight training used steroids/PEDs for artificial advantages. I’d say George Foreman is probably the strongest boxer who never used PEDs (as far as we know, and I’ve never heard any allegations). Him hoisting a cow over his shoulders as a young man, as seen in photos, is just ridiculous. Same as hauling a freaking truck around pulling it by a rope.
If we completely avoid skill and technique, imho the strongest is Carnera and the most durable is Willard.
Ring strenght and outside of the ring strenght are not the same thing. Some boxer were very strong but were unable to use their strenght in the ring like Bonavena or Carnera. Bonavena was pushed and backed by smaller and weaker men but I have seen him lifting the back of S class mercedes benz.
P4P, I'd probably say Saensak Muangsurin. He was an awkward and often cumbersome-looking fighter but he was incredibly strong and had a very big punch.
In a boxing specific sense, as in who could utilise physical strength in a beneficial way with the gloves on, it has to be George Foreman. There will be stronger men who boxed, but their strength wasn’t effective by comparison and may have even worked against them. Whereas Foreman was accurate. George could shove and push out a punch with less technique and do a lot more damage with far less effort than anyone else.
Foreman without a doubt, a fighter who becomes world champion at that age possesses supernatural strength...
Was it ever discussed how Carnera may have had acromegaly? He had similar features to Andre the Giant who had that condition, and being a 6'6" tall Italian man back then would be like being 7' today. Acromegaly creates an excess of growth hormone.
They overpowered the guys in their era, 1000%. But a 6'2 220 "Big" George, doesn't cut it anymore. He has maybe the hardest punch of all time, but strength isn't power. These 280 guys like Zhang, Bakole, Ngannou likely manhandle them around the ring.
Ngannou had proper functional strength from wrestling, surprised the **** out of Fury. Zhang and Bakole walk through punches and hit hard but I've not seen physical manhandling by them, although it wouldn't surprise me if they were proper 'farmer strong', not gym strong. I think Foreman, Liston, and yes, Holyfield were strong guys in any era.