Overall, the best time to use mouthwash will depend on your personal preferences and oral hygiene routine. As long as you use it in combination with regular brushing and flossing, mouthwash wil  be a valuable addition to your oral care routine.
NHS guidance states:  "Using a mouthwash that contains fluoride can help prevent tooth decay, but don't use mouthwash (even a fluoride one) straight after brushing your teeth or it'll wash away the concentrated fluoride in the toothpaste left on your teeth."