For Thai Basil Chicken, the most traditional and authentic choice is to use Thai holy basil leaves. Thai holy basil has a distinctive flavor that adds a unique herbal and slightly peppery note to the dish. It has a strong aroma and pairs exceptionally well with the other ingredients in Thai Basil Chicken. If you cannot find Thai holy basil, you can substitute it with Thai sweet basil, which is milder in flavor but still contributes to the overall aromatic profile of the dish. Regular Italian basil can also be used as a substitute, although it has a different flavor profile compared to Thai basil varieties.