Or maybe this?:

SELECT

t.ProductID

,t.Order_Day

,t.Order_Quantity

FROM (

SELECT

ProductID

,Order_Day

,Order_Quantity

,ROW_NUMBER() OVER(PARTITION BY ProductID ORDER BY Order_Quantity DESC) AS row_num

FROM (

SELECT

SUM(OrderQuantity) AS Order_Quantity

,ProductID

,DATEADD(DAY, DATEDIFF(DAY, 0, OrderDate), 0) AS Order_Day

FROM Transactions

GROUP BY ProductID, DATEADD(DAY, DATEDIFF(DAY, 0, OrderDate), 0)

) AS daily_totals

) AS t

WHERE t.row_num = 1