Manny has defeats too. Sugar Ray Robinson was 85-0 as an armature, 173-19-6 as a pro, 13 of these defeats and 4 draws came when he was well past it. Until Manny or Floyd has names on their record, the same calibre as LaMotta, Basilio, Turpin, Gavilan, Armstrong etc, then they will not be mentioned in the same breath as the greatest of all time.
You have to add to this the fact that boxers didn't get as much rest between fights, didn't get as much time to prepare and heal, didn't have anywhere near as good equipment, didn't have advanced training regimes and didn't have the same nutritional advice or supplements. You have to take all this into consideration when weighing things up
Of course it is possible that Manny, Folyd or Jones Jr could beat the likes or Robinson, Duran, Armstrong etc, all it takes is one punch and anything can happen in boxing

Actually Roy Jones Jr. is the exception to the rule. Even though Roy rarely makes the top 5 all time middle weight list when people discuss the all time greats, most acknowledge that Roy at his peak would have most likely beaten all the greats. You hear names like Ray Robinson, Marvin Hagler, Carlos Monzon, Henry Armstrong, Bob Fitzsimmons and Sugar Ray Leonard mentioned all time along side a host of other middle weight greats, but few if anyone who intelligently and accurately talk boxing fail to acknowledge Roy Jones Jr.'s extraordinary physical skill, speed and quickness. Many experts consider both Bernard Hopkins and James Toney as historical all time great middleweights, and Roy beat both. As a "boxer" many of history's greats had better ability, but it wouldn't have saved them any more than Hopkins and Toney's skill did against Roy. Roy Jones Jr. at his peak was so fast and so quick that he would have won against any I have named, most in convincing fashion.

As for Duran against other lightweights, clearly he was as vastly superior as Jones, and the fact that Roberto moved up in weight to win world titles from 135 to140, 145, 154, 160 and 168 is almost as impressive as Bob Fitzsimmons winning 160, 175 & 200+ titles or Henry Armstrong holding 3 different weight division titles at once. Roberto had middle weight power at 135 and as good as Manny is, Roberto had the ability to go 15 rounds with Marvin Hagle, in short Duran had a chin only Tommy Hearns could dent. Floyd Mayweather would have the speed to give any slugger trouble, much like Meldrick Taylor could, but odds are good Floyd would have trouble against the all time greats like Pryor, Chavez or Duran. Mayweather's fight with Pacquiao will decide this era's best P4P fighter, and both were great at 135 but the aura of history's greats is safe from them for now. The next generation of boxing fan will eventually regulate Duran and Chavez to the status of Bob Fitzsimmons or Henry Armstong, so Floyd an Manny will have their historical moments, but for now they are too recent. Neither Floyd nor Manny would stand much of a chance against history's great Middle Weights, the size factor would be too much for them to over come, Floyd struggled with an old, past his prime DeLa Hoya and Manny has already been stopped twice by lesser fighters, he is not as invincible as he appeared against those who tried to stand and slug with him, a quick heavy puncher like Ray Robinson or Marvin Hagler would be too much for either Floyd or Manny at 160.

As for the heavyweights of today, the 70's were not that long ago, and the Klitschko's may be today's best, but by those standards they are historically similar to Jerry Quarry or Cleveland Williams, they clearly lack the ability and skill level of the great heavyweights of the past.

more knowledgeable people have better answers. but even if you don't know anything about boxing (like myself)- it would always feel logically safe to assume that the older generation has a certain edge over today's best fighters. here, i am not talking about who will win against whom– i am talking about the mythical concept of "boxing greatness". the most obvious advantage of these old timers is that they fought more than 100 fights. big numbers inspire awe. the less obvious reason are the quality of opponents subdued by these old timers. because they fought a lot, it's statistically bound that they would have met also the best in the same era. both of these aspects are not easily attainable noways (2000s). If you are talking about Manny or Floyd (or Roy Jones even) they all do not have fights reaching 100- and you can always scrutinize their records if they faced the other "greats" of their era (because they're still alive). what else can you quantify? (we already have number of fights, number of quality fights). we can also quantify number of belts but over the years, the number of weight divisions changed (the original 8 had grew into all the jr and super this and that to replace catch weight although there are still catch weights). it therefore becomes more hopeless for the most recent fighters to dethrone these older legends (but there is hope because ali had 61 total fights; although roberto duran had 119 but he retired at 50). in the end it's not about losses or wins, it's about the hype that lasts. what writers remember to write about these boxers become the stories that build their legendary status. until now if you ask anyone, who is the greatest boxer that ever lived, without batting an eyelash the answer is Sugar Ray Robinson (even if you don't know why). now this is a real legend, because a lot of people say so. if the fans of manny or floyd or roy jones will continue to talk about their fights and how they won (writers write news columns/books? about them)– that's the only way they could out-shine the old timers. but this has to stand the test of time. so, we have to wait.

Generally because most boxers aren't fully appreciated until years after they've retired. Ali was often criticized during his fighting years, and it's only till many years later that people fully accepted Ali's greatness. Mayweather and Pacquiao will be fully appreciated years after they retire…especially if they retire on top. RJJ, well, he should have retired years ago and his fights now are just tarnishing his legacy.