The best temperature to cook flounder is typically around 375°F (190°C). This temperature allows the fish to cook evenly without drying out or becoming overly browned. However, cooking times can vary depending on the thickness of the fillets, so it's always recommended to keep an eye on the fish and use a fork to check for doneness. The flesh should be opaque and flake easily when cooked through. Adjust the cooking time as needed based on your specific oven and the thickness of the flounder fillets.