Thomas Hearns. It shot out there as fast as a rattlesnake and did just about as much damage to skin (ever seen human skin post-rattlesnake bite?). With that kind of speed and coming at the end of a heavyweight's reach(78 inches) made it as formidable as any in history. I remember back in 1985 when Hearns and Hagler were being measured category by category by one of the mags(The Ring, I think). They scored both men a "10" in the "Jab" dept. but added "Hearns's jab is so devastating that we were tempted to give it a 10-plus. Not only is it effective as an offensive force but is a defensive measure as well. Take away Tommy's potent right and he can still outbox you with the left and capture the decision."
Harry Greb would often stun opponents with his jab, opponents of which he was giving up 15-50lbs! He would stun them with his rapier like jab and they would remain stunned long enough to navigate his way inside, blind in one eye and half blind in the other, then unleash a dazzling flurry. Now tell me what modern fighter could stun a 15-50lbs more fighter with a jab? And i want evidence, no film, i want to see it written on paper
Hearns is the first guy that springs to mind. Conteh as well. Tyson when he bothered to use it could surprise some people.
Monzon's jab was excellent..kind of hypnotic...he would follow it with the right at an opportune time.