I've said Archie Moore before and I'm sticking with him. However, there are some major contenders who are pretty much inseparable from him; Sam Langford, Sammy Angott, Maxie Rosenbloom, Harry Greb... Mickey Walker & Ray Robinson would probably be a slight step below, and slightly below those two would be Roberto Duran, Henry Armstrong and Emile Griffith. Moore's record looks like this: Muhammad Ali Rocky Marciano Floyd Patterson Ezzard Charles x3 Harold Johnson x5 Willie Pastrano Jimmy Bivins x5 Charley Burley Carl Olson Teddy Yarosz That's just the Hall of Famers, not mentioning the rest of the murderer's row (term used loosely); Lloyd Marshall, Eddie Booker, Shorty Hogue, Aaron Wade, Bert Lytell, Jack Chase, and numerous other feared contenders such as Bob Satterfield, Clarence Henry and Nino Valdes. But yeah, there are a few contenders. If anyone wants to put forth another fighter's record, try to keep it in the same format, listing Hall of Famers and the number of times said fighter fought them, and then list honourable mentions afterwards. Makes it easier to compare. If you need to find out if a fighter is in the Hall of Fame, just google his name followed by 'ibhof' and it should come up. For instance 'Jimmy Slattery IBHOF.'
I did forget about McLarnin actually. I think he crammed in the best opposition for the number of fights he had, but there are fighters who were more proven just through facing better boxers overall because of a longer career or more bouts. Sammy Angott, for instance: Ike Williams x3 Beau Jack x2 Ray Robinson x3 Henry Armstrong Willie Pep Bob Montgomery x3 Baby Arizmendi x2 Freddie Miller Lew Jenkins Fritzie Zivic That's just Hall of Famers. Three times against a prime Ray Robinson is enough on its own.
If you mean the man who fought the most champions and contenders, the true answer is Maxie Rosenbloom. Hhascup has fed all the info into a computer. He posted a while back that he didn't believe what he saw, as Rosenbloom was far ahead, and so checked by hand and found it was indeed true.
Not necessarily numerically, although that does help. But someone who faced twenty decent champions might not be as 'proven' as some who faced ten great champions. On that count, Moore's my #1, but perhaps Angott...