Out of the three mentioned I'd order it Hagler, Pac than Pea.. Although I'd probably place Pea in first if it was a head to head comparison. (Not against each other, but other ATGs)
Comparing apples to oranges. Pea was a defensive fighter, pac an offensive fighter. No such thing as THE single best. Each to their own.
If PAC beats Floyd in a fight PAC will probably go to number 1 anyway, his resume is good enough as it is
Of course the Gayweather fans comes out to berate Pacquiao with rumors such as that he's on roids, or that he's too old. He has conquered 7 weightclasses, and he has agreed to taking a bloodtest both before the match and after the match with Mayweather. Also, Pacquiao isn't showing any middleage symptoms of being a roid-user, such as unnatural shaped muscles, acne, etc. And Pacquiao is also one of the most humble and calmest boxers out there. The symptom of roids is that your emotions go up and down, which we haven't seen from Pacquiao. As for best left hand boxers, I would say Pacquiao and Guillermo Rigondeoux, then Hagler.