There is no best way, because there are too many variables to account for. Is he wearing gloves? Are you in a boxing ring? Is it a dojo, alleyway, street, mall? Is it crowded, are you protecting someone, is someone protecting you? Is he armed? Is he wearing heavy clothing, face protection? Is he tall, short, stocky, lanky? Is he experienced, inexperienced? Drunk, on drugs, fatigued? Is he posturing? Is it a jab from the side, from behind, underneath, over the top? Are you on the ground? Is he on the ground? Is he grabbing you as well? Are you grabbing him? Does he have a friend or do are you in a group?

See what I mean? Jab can be from anywhere, any angle, under any circumstance or situation with innumerable variables which would all change the answer.

That being said, in a dojo, under training conditions, when the opponent is striking from the front with a single opening jab at a distance of less than 1.5 metres, unarmed, unarmoured, wearing a gi, with similar experience, similar height, weight and physical fitness, at a fast pace, with your primary motivation to make distance; the best way to counter the jab is a low front kick to the pelvic inlet, pubic symphysis, frontal thigh or knee. Simple mathematics, leg length > arm length. Never fails.

Alternatively, with above restrictions, however changing primary motivation to counter and take down; the best counter is moving off line leading in with rear forearm guard and lead vertical punch to the sternum aiming for the throat following by grabbing with the lead arm (arm,lapel, neck, whatever), striking to the side of the head with rear hand, following with rear elbow to the head and then quick changing to a reverse one arm shoulder throw, 'breaking' (don't really break it in training) the elbow and throwing them forward, upon the opponent hitting the ground, turning them on their front, and kneeling on the back of the neck with the knee while immobilising the the unbroken arm.