Mayweather lost to Castillo and Maidana, but his corrupt officials had his back. Ali was old and faded when he lost to prime Spinks. Norton was a top heavyweight in his prime. If Mayweather fought DLH, Pacquiao, Tszyu, Trinidad, Forrest, Margarito, Williams and Cotto in their primes, he'd have a lot of losses.
I don't get why people say this. Both fights were competitive but Mayweather winning was not a robbery. People misuse that word a lot.
For the question I'll say Floyd; He has more good wins, better overall wins, beat 3 ATG, and has success in multiple weight classes, and has been able to consistently beat fighters who were bigger then him. Don't really see the argument for Louis just looking at it statistically
The argument for Louis is straight-forward. He is, in all divisions, the most dominant champion the sport has ever seen. I'll use Floyd's rationale here of, 'Number's don't lie.' Twenty-five title defences. However, it doesn't stop there. Dominance. Had Mike Tyson laboured to points decisions he'd be rated nowhere near as highly. Performances matter. Now look at what Joe did to his opponents, particularly in rematches. Was anyone better in rematches in the whole of the sport's history? I'll admit the question is rhetorical. 11-0 (9 KO's) in rematches, and the two guys who went the distance did so with a post WWII Joe Louis. Louis is one of the few people who put in performances that outshone Floyd. The manhandling of the giant Primo Carnera, the four round KO of iron-chinned Max Baer, the 124 second destruction of Max Schemling while the world watched. Dazzling against Corrales and Gatti isn't quite the same.
Floyd has a lot of name fighters on his record. His most decorated were past prime. Louis' KO over future champ 'Jersey' Joe is about as good as anything on Floyd's record. Further, Louis crushing his opponents made them look worse. Always happens with punchers.