We’re talking about guys who would fight literally anybody and risk it all to win every time... boxing has seen quite a few fighters like this. One name that comes to mind for me is Sam Langford. What about some others ?
Archie Moore, Sugar Ray Robinson, Rocky Marciano, Carmen Basilio, Benny Paret, Muhammad Ali, Joe Frazier, George Foreman, Ron Lyle, Bennie Briscoe, Bobby Chacon, Matthew Saad Muhammad, Larry Holmes, Leon Spinks, Evander Holyfield, and Bert Cooper are the most courageous fighters I've ever seen, no matter how successful some of them were.
Few braver than Fritzie Zivic. He fought: Sugar Ray x2, Billy Conn, LaMotta x4, Armstrong x3, Ambers, Angott, Jenkins, Jack, Montgomery, Cochrane x3, Jadick as well as top-notch contenders Burley x3, Booker, Leto and Azteca x4. Wow!! So impressive was the weight he gave up to LaMotta to engage him 4 times.
Yeah, but it's missing Sammy Angott. The guy fought Ike Williams thrice, SRR thrice, Henry Armstrong, Willie Pep, Fritzie Zivic, Beau Jack, Lew Jenkins, Bob Montgomery and even more HOfers.
I forgot about LaMotta! One reason he didn't come to mind is because the film we have of him isn't the best.
I'm thinking more of styles and performances, rather than resumes, although that is indicative of a great champion.