Write SQL query to solve given problem: State the order number where Qualitest ordered the highest product quantity.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T1.OrderNumber FROM `Sales Orders` AS T1 INNER JOIN Customers AS T2 ON T2.CustomerID = T1._CustomerID WHERE T2.`Customer Names` = 'Qualitest ' ORDER BY T1.`Order Quantity` DESC LIMIT 1