The jab, maybe the most versatile punch in boxing.
It's used to punish, distract, set up combos, get inside, stay outside, keep scoring while backing up, and an excellent range finder.

In my opinion, young George Foreman had the most punishing jab I've seen. It was a fast, accurate, thudding, and head/neck snapping weapon.
In my opinion, Wlad has the best jab today, he uses it constantly during a match. He could probably win some fights with nothing but his jab. That might be a small exaggeration. LOL

What are your picks, and why doesn't everyone try to develop a good jab?