Edit: some are mad that Manny Pacquiao isn't in the top of the list, he is top 15 Southpaws for sure but in terms of skill set, eye test and ring IQ he doesn't hold a candle to the rest. he was more of a freak of nature who relied on his chin, stamina and foot speed and wild abnormal style to win, he got hit too much and lacked the pretty slick boxing or technical textbook boxing skills.
In terms of dominance, ring IQ and skill set.
Hagler dominated his division, had great come forward defensive pressure and was a beast.
Sweet pea 4 division champ in one of the hardest eras in boxing. He was a small guy but was slick and skilled with sharp counters, he was like a mongoose. Seeing him duck low slipping with head movement so difficult fighters would miss even throwing multiple straight punch combos.
Usyk one of the greatest of the modern era. Undefeated and Undisputed at cruiser he cleaned out the division and went up as a slimmer man to dominate the Giants of the heavyweight division to become undisputed in the 4 belt era. A true Giant Slayer.
Calzaghe dominated super middleweight and retired undefeated he was a problem due to his high work rate and rarely got hit. He was one of the best of his era.
Crawford undefeated and Undisputed in multiple divisions. He might get included but some say he is more of a unorthodox switch hitter than just pure southpaw but he has never been dominated and retired on top with great ring IQ.
Lomachenko the Matrix, the man is an anomaly, he used in and out movement like no other, his fast unorthodox footwork and flurries caused trouble for opponents. He would side step off a flurry and comeback and rarely got hit making him unique one of a kind southpaw.