I would start here: http://www.boxingforum24.com/showthread.php?t=124003 http://www.boxingforum24.com/showthread.php?t=83290
Whitaker, Hagler and id even throw Manny in there..Hagler didnt always fight southpaw but when he did it was perfect!
Whitaker. He was something beyond a technician. A stylist with great timing, rhythm, reflexes, innovation, adaptability, and complexity. He's one of only 5 fighters who peaked after 1960 in my top 30, not to mention one of only 2 southpaws. Two spots after him is Tiger Flowers. Pacquiao will have an argument someday, depending on what happens over the next 2 years. Lew Tendler should always be mentioned here...
Tendler definitely ranks highly here, and Whitaker was perhaps my favorite that i've seen, but I agree Nunn is an excellent candidate.
"Ultimate Book of Boxing Lists" list: 1. Hagler 2. Whitaker 3. Pacquiao 4. Flowers 5. Callaghan 6. Moorer 7. Marvin Johnson 8. Tender 9. Tarver 10. Saldivar Bert Sugar & Teddy Atlas, with numerous contributors. Highly recommended.
It is Hagler. All the other great southpaw champions were great BECAUSE they were southpaws. Hagler was an all rounder. Box-fighter, boxer, slugger, southpaw box fighter, southpaw boxer AND southpaw fighter. He was as good both ways around. That is why he is the best.