What is the best over-the-counter treatment for old acne scars?

Nothing will work: I'm really sorry your acne was neglected and you had scarring. If you still have active disease, get it managed by a competent physician. Unlike in the past, you're now basically an adult and can pick up the telephone and schedule an appointment. Until you get seen, use benzoyl peroxide as tolerated. Your scars can be managed with filler, laser, or microsurgery if you wish.