Best sleep aid--diphenhydramine hydrochloride or doxylamine succinate?

Both commonly used. Both of these are commonly used over-the-counter. They are sedating and help people fall and stay asleep to a greater or lesser degree depending on the person. Often tolerance develops if the person takes one or the other regularly. Melatonin is another otc medication that can help with sleep. Be sure to practice sleep hygiene -- structuring your sleep, avoid lying long periods not sleeping, etc.