There's no fight footage of Harry Greb. Only some brief training where he doesn't look all that impressive but on paper he has a resume that could be argued is as good as any in history. Footage of Sam Langford is sparse also but his resume is also incredible and was essentially a welterweight good enough to have been champion all the way up to heavyweight. A victim of his era. Those two surely would be in a top 5 of all-time based on resume.
I think along with Langford and Greb, you've got to put Willie Pep. pep was 134-1 entering the Saddler fight and that was already after he had a near fatal plane crash. and had started to show signs of slippage. He was so good that he managed a re-match win over Saddler, himself a ridiculously great fighter arguably #2 in featherweight history. The footage of a prime Pep is limited and what's available period show a perfect boxer, with an incredible defense, speed and reflexes. Henry Armstrong I think for sure has to be in the top 5. Only man to simultaneously hold three world titles in an era of 8 divisions, 1 champion per division. Made a record breaking 19 welterweight defenses and had a style that would simply just overwhelm most in history and then you've got Sugar Ray Robinson. Like Pep, posted some insane numbers and his style still holds up today when assessing tape. Many from that generation say he looked even better as a lightweight than as a welterweight meanwhile most the footage available is as a middleweight where he's arguably the best ever despite that being past his best and giving up advantages in age and size. His resume is incredibly deep and his highs incredibly high.
Those would probably be the five greatest IMO. As far as order I'd go 1. SRR 2. Armstrong 3. Pep 4. Langford 5. Greb
As far as some of the contenders for best then Benny Leonard may have to be included due to being way ahead of his time with a very technical boxing style. I'd have him somewhere in the top 10. Roberto Duran was the ultimate fighting machine. At his best he was incredible though I think Carlos Ortiz may have had the measure of him prime for prime at lightweight. Duran undoubtedly has the greater career due to some of his huge moments above 147. Duran I could see being top 5 with no complaints. Ortiz was a vastly underrated boxer who could do it all. He could box, he was strong & was as smart as they come. He wasn't a one punch KO artist nor was he blazing quick but but his KO over Flash Elorde was chilling and his masterpiece against Ismael Laguna in the rubber match when Laguna's youth, speed and legs were widely touted to be too much for an aging Ortiz. That's one of my favorite fights. If I had a time machine that's one of the fights I'd want to have seen live.
Eder Jofre was a perfect fighter. Technique, patience, poise, power, balance he had it all. It's really a shame that there's only been four of his full fights available and highlights of around 15 or so more. He really deserved the decision over Harada in the first fight and was hurt by the same day weigh-in's of that era because he was killing himself making 118. In the modern era he'd have had a chance to dominate 122 as well. It's a testament to his skills and smarts that he went away and came back in his mid 30's and embarked on a 25-0 unbeaten run including an excellent win over a stylistic nightmare in Jose Legra. Ezzard Charles was another complete, almost perfect boxer with a tremendously deep resume. I find it interesting when reading views of historically great boxers from publications in the 60's, 70's and 80's he's not given the kind of kudos the likes of Moore, Louis, Pep etc; are given. Perhaps due to not holding the 175-title and a relatively short reign as HW champ? His resume has grew in time.
That's sort of my alternative five. Another name I want to mention as far as p4p and best is Jose Napoles. I'd rank his career probably around 30-40 all-time but just from a technique standpoint he's right up there with the greats. He suffered first from being avoided at 135 which would have been his ideal weight and then from brittle skin at 147 where he was undersized. In the modern day and age he'd have likely got a shot at 135 and a chance to dominate 140 before winning at 147. Considering he was a major booze hound also he still had an amazing career. I think he's one of the "best" ever as in arguably top 20 but "best" and "greatest" are different. For example I think he's the best Cuban ever but as far as greatest he may be third behind Gavilan and Chocolate.
Floyd Mayweather is another that will grow with time I feel. Not to T.B.E status that his young and overzealous fans try to force down our throats but slowly and steadily like Charles. I think he'll end up being viewed in the top 20. Sugar Ray Leonard is up there with the very best of them. He had it all. He just suffers from basically being a part time fighter for what should have been his prime. He got some amazing wins but his numbers are lagging far behind the likes of SRR but I can accept him being in your top 10 of all-time because he was that good. You have to mention Muhamad Ali too and Joe Louis.. That's another five.
Sorry about being long winded. It's such a hard category. It's very difficult to separate greatest from best and compare different era's. Hopefully somebody found my post enjoyable

