Sam Langford. From Lightweight to Heavyweight, no champion wanted to risk their title against him. Most intimidating of more modern times, I go with Liston.
Although I don't know ALL fighters in the history of the sport (lol), I'd say Liston, followed by Foreman.
Liston would be up there. His ties with the mob, his demeanor, his criminal record, his surly glare. And his punching. Can't forget that.
I'd add Joe Louis and Sugar Ray Robinson, in their heyday. And, as an amateur, every simon pure **** a brick at the thought of facing Mark Breland. I was in countless lockerrooms, and it was always the same scenario: dead silence. Every kid looked at Mark's opponent like he was a condemed man. It was the Joe Louis syndrome: they lost the fight before they ever got to the ring.
This guy who worked in the warehouse I got a summer job in once, boxing up pairs of Doc Martens. He thought that Slash was "better than Jimi Hendrix", and actually liked Noel Edmonds.
I can believe the Breland part, but as far as adding Louis and Robinson? Everyone seemed to want to get in line to beat Louis due to him being black and they wanted to dethrone him. Robinson had a long line of fighters that believed that could beat him. My pick would be Tyson, Liston and Big George.
Whatever you may have heard or read, B, about fighters jumping at the chance to face Louis or SRR, that was more promoter and flack chest-thumping than the fighters. All sellin' wolf tickets. In the hardcore gyms in the '40s, the mention of Louis and Robinson sent shivers through every fighter and trainer. They knew better than anyone how lethal they were. When the fighters got the shot. I watched them in training, and the eleventh hour, when they left the dressing room. It was always the same; they were dead men walking.
Tie between J. Johnson and Ali. Because not only were they the Greatest, they were defiant Black men in eras when Water Fountains were still segregated.