Gennady Golovkin. His jab is one of his best assets of his success yet still doesn’t get appreciated enough for it. Its also one of the reasons why most people feel that he won the second fight against Canelo as well.
Another one I think would be Bobo Olson,his jab was rarely seen but you can see in his second fight with SRR,that he outjabbed sugar on a lot of occasions.
Not sure if it's underrated but Sot Chitalada had an excellent power jab that he could use from both stances.
McCallum isn't too underrated in this day and age, but I don't hear his jab get talked about too much. He'd throw it to start/end most combinations, and he'd keep it coming no matter what position he was in or where he might be. Resetting? Jab. Exerting pressure? Jab. Taking a step back? Jab. He's mostly well known for his counters- Right hands to the head and variable hooks to the body, but that jab was a thing of beauty, too.
I think Robinson’s jab was his Achilles heel… Robinson is Robinson but his jab wasn’t his best asset, compared to the rest of his kit… so so.
Mike Tyson routinely out jabbed much taller men with substantial reach advantages. So did Dwight Qawi. Both men demonstrated that good jabbing is much more than having a height and reach advantage. Timing and speed are just as important, if not more so. Aaron Pryor also out jabbed taller fighters in a few of his fights. Joe Frazier used his jab very well. And was much more than his patented left hook. Also Roberto Duràn jab is usually overlooked. Most great fighters were very good to great at jabbing. Though it's often overlooked because of their bread and butter punches or tactics.