Ali. As this thread does not specify P4P I take my choice from the Open division in boxing, and the division that counts for the most - the Heavyweights.
Whitaker had much more variety than Pep as well. Pep's defense was all about movement, and slipping under punches. Whitaker had much more variety. Because Pep fought around 60-70 years ago, and had over 200 fights, many people on here will tell you he was better than Whitaker. You could make a case he deserves to be ranked ahead of Whitaker P4P. But when it comes to skills and what each fighter could do in the ring, Whitaker without a doubt in my eyes.
Sweet Pea, et al. I think that you underestimate Pep. In terms of sheer skill, there has been no one like him, except for Pernell and even Pernell was a relative novice when you look at activity. A 135-1-1 streak is sickeningly great. Pernell was like 32-1 at his best. I take serious issue with the idea that guys with under 50 fights can be compared with guys like Pep. People casually say "oh but he fought no one" -as if. Pep stepped into the ring and put his life and health in jeopardy 240 times against professionals: Jackie Wilson, Chalky Wright, Willie Joyce, and Paddy DeMarco, and Sandy Saddler are among his wins. You also forget the testimony of those who saw him and his opponents. Many compared fighting Pep to trying to "put moonbeams in a jar" fighting a man in a hall of mirrors, or chasing a shadow. One of them said that it was like "trying to stamp out a grass fire." Pep was a beautiful boxer who would make his man miss by centimeters -his defense was usually inside the perimeter. Many celebrate Ali's "defensive skill" -it couldn't compare to Pep's. Pep was a textbook fighter who wrote his own because he was too advanced for it -it wasn't Ali-Jones speedy unorthodoxy -it was calculus 3. Angott had his number -and I'd be willing to bet the house that Angott would have had Pernell's number just as much. Angott's style was the foil for small innovators because it was designed for disruption, frustration, and smothering. An Armstrong got by him because of the strength and power factor. Pep was no Hercules in there (and contrary to a few confusing opinions out here, neither was Pernell). You seem to dismiss Pep's comeback from the plane crash. I think it is among the greatest boxing comebacks in history. He was never supposed to walk again, never mind fight... a broken back, paralyzed from the waist down for a time, and in a body cast for 5 months. The fact that he actually offered his greatest masterpiece in beating Saddler after the crash is shocking. He fought Sandy, one of the greatest to come out of Boston, MA (my home town) for the second time in his 140th fight. Fact is, Saddler probably had the Pep puzzle solved too due to his style -Pep shouldn't have beat him at all. So, if this means who was the "greatest", pound for pound, I would take Robinson. Sometimes conventional wisdom is right. If you mean who was the greatest "boxer" who ever lived, meaning who had the highest level of skill, I'd proclaim Pep. He was superior to them all...a savant technician.
I'd say 42-0 to be honest. I couldn't justify Ramirez and Chavez if i was paid money to. Some will debate the DLH fight but i'll make that loss the cutoff.
Pernell was great and I don't deny that -just to be clear. I see them as similar in their skill, but would argue that Pep's was more honed because Pep was more experienced. Pep was in the ring 6 times as much as Pernell was... Now, you would counter with the level of competition and I would agree that while that is true, the chasm between them in terms of comp is not as great as it is made out to be. Alot of posters out here dismiss Pep's record and yet know nothing about for example, Chalky Wright. As for Sammy Engotti... his style would be problematic for Pernell. Again, Armstrong beat Engotti -due to strength and power. Beau Jack did as well, but not without stretching out a bit himself. Pep fought him once and left him alone. Some say that it was a hometown gift, but I think that it was a styles problem. Angott was no slouch -he beat Ike, Pep, Montgomery, Arizmendi, Freddie Miller, Johnny Bratton, Tendler ... he stopped the rise of Lenny Mancini (yep, Boom-Boom's pop). The man was strong and had far more experience with elite fighters than Pep, Robinson, and Pernell combined! Angott was a great wrestler and perhaps the best of the maulers ever. He was very good inside and not bad outside as well -he could bang and he could take it -being stopped once in a 130 fights. The Armstrong fight was actually very close, reportedly. I don't see Pernell being allowed to innovate and shimmy and shake with him because "the Clutch" would be on him like a cheap suit.