Some good call's on this already especially the HW's. From the UK Frank Bruno had a fantastic jab,ramrod hard very underrated.
Im not asking for snapping jabs like alis, im talking about heavy jabs that can knock an opponent down or hurt him. Kovalev Quartey and Bruno are good suggestions.
Recently did a thread on the effectiveness of Monzon's jab...but at 160, I think Hagler's southpaw jab was the most punishing jab...
Allow me to expand then. Strictly power wise - Ike Quartey is very strongly in the mix P4P. Surprised Cooney not been mentioned so far. His jab busted peoples features up. Hearns has the best jab i have ever seen, and it's plenty hard. Lewis had a heavy jab when he leant on it, and every punch Shavers threw was heavy. The guy i most want to bring attention to however is Bob Foster as it's a crime that he has not yet been mentioned. His jab at 175 was an absolute spear. Many a time it busted opponents up inside the first handful of rounds. Even at heavyweight it busted Ali's face up, no mean feat. He most often fought left hand low like Hearns, so that jab was ripping up into faces from waist level. One of the greatest jabs ever, and among the hardest too.
Good to see Michael Spinks at LH mentioned. A lot of them are obvious - Holmes, Liston, Foreman and Ike Quartey (along with Spinks) were the first to come to mind.