Generally if you are an orthodox fighter and so is your opponent, you will lean to the right to slip his left jab which sets you up for either an overhand right to the head, right to the body or depending on how much space you have left possibly a left uppercut or left hook (depending on your footwork also). An overhand right is probably the best but if your opponent has a decent defence his shoulder should be covering it. Try parrying the jab rather than just slipping it, so as it comes in knock it with your right hand to your left - this should open up his left side more for your right hand.
Jab, right hand, left hook, jab to the body, right to the body, whatever. Depends on where you move, how quick you are and what your reflex action would be.
i'm a southpaw and i love to counter the jab with slip + right uppercut between my opponents hands. if i were a orthodox fighter i think i'd use overhand right or right upper.
slip a to the outside throw a right hand and step to the side and go over under with a left hook to the body or head to the inside slip left to the body short right uppercut