Excluding switch-hitters...….e.g., Hagler and Chiquita Gonzalez for example do not qualify. Strictly southpaw. Is it Saldivar? Tendler?
I would have said Hagler but if he's excluded ... Pacquiao was pretty good. Whitaker was pretty good.
Skill-wise and talent-wise, Watanabe is right up there. Though he was actually naturally right handed, as was Saldivar.
My inclination is to also go with Whitaker, though Tendler has a strong claim too. More remarkable might be that Tendler fought in an era when they routinely switched them to righty. Wonder why he never switched.