Hagler would be my choice as well. His jab tends to get underrated by the casual boxing fan. For sheer snap and power, his is tough to beat. It was basically the equivalent of a cross, being as Hagler was a converted southpaw. He put his entire body into that jab.
It's certainly the GOAT welterweight jab, and way up there p4p. Obviously heavier guys have had harder jabs in a straight-up sense.
Hagler jab was very good, esp' with his bout with Sibson, ODLH jab was strong as well, maybe some thing about your stronger hand being your jab side. I know that my jab is as strong as my right, being a natrual lefty. A jab like Monzon's was very good as well to set up the right hand, which is most of the job of a jab, to set things up. If that counts.
Some say Larry Holmes' left jab hurts as much as many other's rights. Saying that Ike Quartey's jab is very impressive.
In a thread about the most effective/versatile jabs, Monzon's would definitely be up there, among others. Hagler's just takes the cake for me as far as the most damaging.