Can you tell me what is the best OTC sleep aid to use from time to time?

There is no "best".. The best one is one that works for you and doesn't cause any problems. The most common over the counter sleep aids have diphenhydramine, and this is in Benadryl and unisom. Nyquil has a sleep medicine (zzquil) that also has diphenhydramine, but it also has alcohol. You could also try melatonin is something you can buy in the store that can help with sleep.