Since the sport began, boxers have stood in a staggered stance (one foot in front of the other). The main reason for this is simple - you want to be able to move in all directions equally as well as be balanced in all directions. If your feet were together or completely square and someone were to push you straight into the chest, you would fall backwards.
There are other reasons to have a staggered stance and one of them has to do with punches. We traditionally like to have our power side (dominant hand) furthest from the target. Provided that the punch is thrown properly and continues to accelerate, this means it will deliver more power the further it travels. You can also put more of your body, more rotation, into the punch. This is the same reason that people stand in a similar type of stance when they want to throw something as hard/far as they can.
Thus, for the 90% of us who are right-handed, we would stand with our left foot forward, right foot back. For the 10% of lefties, otherwise known as southpaws, they would do it the opposite way - right foot forward, left foot back. This disparity tends to give southpaws a little bit of an advantage over right-handed, or orthodox, fighters. Imagine you have 10 people training together. Based on the average, 9 of them are orthodox fighters and only 1 is a southpaw. That one person is going to get a ton of experience fighting against righties whereas the orthodox group is only going to have one southpaw to practice with. Fighting against them will be awkward and different because it is not what you're used to. But it is what they are used to.
This made it difficult for a lot of land-handed boxers to get fights in the early days. Probably still does in certain cases. If you are an up and coming fighter and trying to manage your career wisely, you don't want to take any chance that you might take a loss to someone who may not necessarily be better than you, they just fight in a style that you aren't used to. Once a southpaw makes a name for themselves, it is a different story but getting over that initial hump could be tough. Eventually, plenty of lefties did make names for themselves and went on to become some of the greatest fighters of all time.
Here are my Top 5, plus a longer list of currently active boxers who may easily become an all-time great southpaw, once their career is over:
Honorable Mention - Rocky Balboa
Despite the fact that Rocky was not a real boxer nor even a real person, I believe he deserves special mention here. His character was indeed a southpaw (unlike Jake Gyllenhaal in the movie actually called "Southpaw") and it was woven into the story. In fact, many average people were introduced to boxing through the Rocky films. Some became fans of the sport, some didn't, but those movies became part of our pop culture. In fact, even just the term "southpaw" got into our modern vernacular because of Rocky. And a case can easily be made that Rocky may be the most important boxer of all time, outside of Muhammad Ali when you look at the cultural impact. So this is for you, "Italian Stallion" and now onto the real fighters...
#5) Hector "Macho" Camacho
There are others who could challenge for this spot, like Vincent Saldivar, Tiger Flowers and Gabriel Elorde, but they all fought many years ago and I can't speak too intelligently about how good they really were and how good their competition was. Then there are more modern guys who I did see fight, like Prince Naseem Hamed and Sergio Martinez but I would put Macho ahead of them because he was a relevant fighter for a longer period of time and he was a hell of a lot more exciting than all those guys. Had probably the best hand-speed for his size, finished most of his fights in his prime and only got knocked down one time after many, many fights. He showed heart and toughness when he was getting pummeled by a younger, hungrier, and all-time great himself, Julio Cesar Chavez. Plus, before personal problems and drug abuse started to take its toll, his style was very fun to watch and he brought a joy into the ring that no one else really ever has.
#4) Joe Calzaghe
One of the strangest styles of boxing you will ever see, Joe Calzaghe threw fast, slappy punches that
would still manage to knock people out and overwhelm them with aggression. Kind of like the Diaz brothers' (also southpaws) "Stockton Slap." He won titles in two different weight classes and is largely recognized as the best Super Middleweight of time. Joe retired undefeated, at 46-0, with wins against Chris Eubank, Bernard Hopkins and Roy Jones Jr. I might have put him a spot higher on my list if he performed better in some of his seemingly easier fights. And he might be more of a household name if he fought more in the USA but he mostly competed in the UK, particularly in his native Wales.
#3) Pernell Whitaker
"Sweat Pea" Whitaker is undoubtedly one of the best defensive fighters ever and was truly a master of the sweet science. His head movement and footwork was as slick as they come. From the mid-eighties to the mid-90s, Pernell was mostly unhittable. Hardcore boxing enthusiasts might rank him even higher than I would because he was such an amazing tactician and that makes them sound smart but for me, who is not as cool nor smart, I need more fight finishes (the majority of his victories were unanimous decisions) as well as signature wins to consider him the top of the tops. But still, he is awesome. Champion in 4 different weight classes and defended his unified lightweight belt 6 times, which I think is still a record.
#2) Marvelous Marvin Hagler
The consummate boxer-puncher, Hagler had speed, power and a granite chin. In his 67 fights, he was only credited with being knocked down once and it was pretty clearly a slip. He had an 84% knockout-to win percentage, highest of all undisputed middleweight champions and he held that same belt for 6 and a half years, which is 2nd longest of all time. The only loss he suffered in the last 11 years of his career was his final fight against Sugar Ray Leonard, which was a split decision that is highly disputed. Even as his skills began to decline with age, he still fought in the greatest, most exciting fight of all time against Thomas Hearns in 1985. He also fought in an era I grew up in, with the 4 Kings, and defeated two of them clearly (Hearns and Duran), potentially defeating the other (Sugar Ray), depending on how you saw that fight. Marvelous was also held back by what I talked about earlier, where opponents wouldn't take fights with him because he was a southpaw. He was a top-ranked middleweight for years before he finally got a shot at the title. Once he got it though, he never looked back and went on a historic run of 12 undisputed middleweight title defenses. Plus he was just a baaaaad man, and that always wins points with me.
#1) Manny Pacquiao
Of course I looked up how other people rank the best southpaw fighters online before writing this and many of these choices are pretty universal and obvious, but what surprised me was how few people had Manny Pacquiao as #1. Bert Sugar was the only person I could find who agreed me. I've seen him as low as 4 on some lists and on Reddit posts not even get mentioned. To me, it's not even really close. This is backed up by statistics, the quality of opponents he fought, and the eye test.
First, let's talk statistics. He is the only boxer in history to win titles in 8 different weight divisions. 8! If you were to win titles in 4 different weight divisions, you would likely be an all-time great and you would have half of what Pac Man accomplished. He started his career at 108 pounds and eventually won a title at Super Welterweight, which is 147-154 pounds. That is pretty insane. He is also the only boxer to hold world titles in 4 different decades, he was the oldest welterweight champion at age 40, and the first boxer in history to win the lineal championship in 5 different weight divisions.
Of course, none of this matters if he fought a bunch of bums. But no, he fought and defeated bona fide hall of famers. Marco Antonio Barrera, Juan Manuel Marquez, Erik Morales, Oscar de la Hoya, Ricky Hatton, Miguel Cotto, Shane Mosley, Antonio Margarito, and Timothy Bradley. Who has a resume like that? Supposedly the best ever, TBE, Floyd Mayweather Jr. ducked Manny Pacquiao for years. When Floyd finally fought Manny in 2015, Pac Man was definitely past his prime but I still don't think Mayweather won that fight.
Yes, there were some losses too. Besides the Mayweather fight, there was a decision loss to Erik Morales and Timothy Bradley. Both of which, Pacquiao would avenge. Most notably though, it was a vicious knockout by Juan Manuel Marquez that would be the only potential stain on his resume. It was at the very end of the round, Manny walked right into it, and it was the 4th time he fought Marquez, scoring a draw and close decision victories before. I can't imagine any boxers now fighting someone 4 times, let alone a hall of famer like Marquez. To me, a loss like that doesn't weigh as much when you are constantly fighting killers.
Then there is the eye test. Pac Man had some of the best hand speed and quick bursts we've seen. He was aggressive and exciting, kamikaze like in his younger days, but his punches were also extremely accurate and his defense, footwork and fight IQ generally kept him safe in the ring. I truly can't understand why he doesn't get more credit. To me, he is the unquestioned best southpaw ever, the best boxer of the 2000's decade, and one of the greatest of all time.
At Least 1 of These Guys
It's hard to put current fighters in historical context while their careers are still active. They may go on to achieve greater success or maybe they fall off a cliff. But I think it is safe to say that at least one of these guys on this list below (in no particular order) will make their way into the Top 5 best southpaws of all time or at least the Top 10.
Oleksandr Usyk
Vasyli Lomachenko
Terrence Crawford
Gervonta Davis
Shakur Stephenson
Josh Taylor
Errol Spence
Amanda Serrano