Henry Armstrong would probably get my vote for #1. Les Darcy, Dick Tiger, and Dwight Qawi are honorable mentions. Barbados Joe Walcott probably belongs among here as well, but that's just going by accounts of him.
Armstrong surely. Probably Langford aswell...Fenech, Williams, Duran, Wilde Manny P gets a shout for me aswell...He is as strong as any of the guys he has fought at these higher weights and I remember he apparently used to manhandle welterweights when sparring for fights at feather.
Terry Norris bench pressed 325 whilst weighing 155. Strong mother****er. For proven applied strength in combat, Monzon was VERY strong. And Foreman at 500 pounds would still be pound for pound in the running. At 220, he was just scary.
Well obviously overall... I'd say Foreman was the strongest fighter of all time. P4P....Qawi @ 175 and Tiger @ 160 immediately came to mind.
Armstrong peaked at 130, yet regularly outmuscled welterweights and even a natural jr. middleweight, while never really more than a jr. welterweight himself. That's damn impressive.
Rocky Marciano is an obvious answer...forget that he's dwarfed by modern day heavyweights, he was p4p one of, if not the strongest fighters of all time...a fact that was almost universally agreed upon in most most boxing writers and people in the sport.