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The top three answers you are likely to get when consulting historians/hardcore boxing fans is Pernell Whitaker, Marvin Hagler and Manny Pacquiao.

A simple statistical comparison tells us this:

Marvin Hagler

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  • Was a middleweight his whole career.
  • Was the Champion (I’m talking about lineal here) for 6 years, 7 months (1980–1987).
  • Made 12 successful defenses.
  • Was 13–1–1 (12 KOs) in title fights.
  • Fought 3 Hall-of-Famers (Hearns, Duran and Leonard) and went 2–1 (1 KO) against them.
  • Duran is generally seen as belonging in the top-10 pound-for-pound all-time; Ray Leonard usually cracks the top-15.
  • Beat at least 14 men who were rated in the top-10.
  • No pound-for-pound ratings available.
  • IBRO ranks him as the 6th best middleweight ever; The Ring had him at #4 (2004)
  • Career spanned from 1973–1987 (Nearly 14 years)
  • Finished at 62–3–2 (52 KOs).
  • All but one of Hagler’s losses/draws are considered controversial.
  • Hagler, stylistically, is widely considered the most well-rounded Middleweight Champion of all-time. He could box, bang, had wonderful technique, a legendary chin, good defensive skills, fantastic work ethic and an indomitable will.

Pernell Whitaker

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  • Fought from 135 to 160.
  • Was the Champion at lightweight (1990-1992) and welterweight (1993–1997), making him “the man” for a little over 5 years.
  • He amassed a total of 13 successful defenses.
  • Was 13–1–1 (3 KOs) in title fights.
  • Fought 4 Hall-of-Famers (Nelson, Chavez, De La Hoya and Trinidad) and went 1–2–1 against them.
  • Nelson is seen as one of the two greatest African fighters (between him and Dick Tiger); Chavez is often known as the greatest Mexican pugilist ever; De La Hoya is seen as one of the better 135–147-pounders ever; and Trinidad is normally seen as a top-5 Puerto Rican and one of the better welterweights of all-time.
  • Defeated at least 9 guys who were rated in the top-10.
  • Was pound-for-pound #1 from 1993–1995.
  • IBRO rates him #7 at lightweight; The Ring last had him at #3 (2001). IBRO has him #17 at welterweight; The Ring ranks him at #19.
  • Career spanned from 1984–2001 (16 years)
  • Finished at 40–4–1 (17 KOs).
  • Two of his losses are considered controversial, as is his lone draw. The draw to Chavez is overwhelmingly seen as a b.s. political decision, with most having Whitaker winning handily.
  • Whitaker was one of the greatest defensive fighters ever. He was a slippery southpaw with a beautiful jab, underrated body work, great timing, a sturdy beard, very good stamina, and had no qualms about facing the best of his day.

Manny Pacquiao

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  • Fought from 112–160.
  • Was the Champion at flyweight (1998–199), featherweight (2003–2005), welterweight (2016). He was Champion for under 3 years.
  • He had 4 total defenses.
  • Was 6–1–1 (4 KOs) in title fights.
  • Fought 2 Hall-of-Famers (Barrera and De La Hoya) and went 3–0 (2 KOs) against them. Since he is the most recent of the bunch, this number will expand at least to 6, with Morales, Marquez, Mayweather and Cotto being locks, and possibly a few more joining. That would bring his win-loss-draw to 8–2–1 (5 KOs).
  • Barrera, Morales and Marquez are viewed as three of the best Mexican pugilists to ever wear a pair of gloves; Oscar De La Hoya is noted above; Miguel Cotto is seen as one of the greater Puerto Rican fighters; and Mayweather is arguably the best of his era.
  • Beat at least 9 top-10 ranked opponents.
  • Was pound-for-pound #1 from 2008–2011.
  • No rankings available, as his career is still in progress.
  • Career spanned from 1995–Current (22 years)
  • Is 61–7–2 (39 KOs).
  • Two of his losses are considered controversial, the SD to Bradley and the UD to Horn.
  • The “Pac Man”, as he’s called, started off as a raw puncher but grew into a savvy speedster who could outbox bigger men, even into his later years. Initially, he relied heavily on his straight left and combination punching, and even through those relatively narrow dimensions he was able to overcome legends. Eventually, he incorporated a good right hand, solid body work, smart footwork and a greater ability to adjust.

Conclusion

While the listed data lacks some nuance, it gives an overview as to why any one of these boxers could be placed as the greatest lefty to ever compete in the gloved-game. If you prefer divisional dominance, Hagler or Whitaker is likely your man. If you appreciate more weight-jumping fluidity and longevity, Pacquiao can be that guy.

Notes

To help level the playing field, I used an “original eight” context, meaning, I treated Whitaker’s and Pacquiao’s resumes as if they were fighting in an era of eight divisions. Therefore only lineal fights and rankings were counted. The reason being is that both men’s CVs become exaggerated if I add in the junior/super classes and the plethora of “world championships” that have saturated the sport.

I also used Cyber Boxing Zone’s lineal encyclopedia to establish championship reigns, as it’s the best-kept.

Top-10 opponents are counted only if they were rated at the time they fought.

*This post is a revised version of an answer originally featured in Quora