List the top five most costly products in 1998.
most costly refers to max(add(unit price , Freight)); in 1998 refers to OrderDate > = '1998-01-01 00:00:00' AND OrderDate < '1999-01-01 00:00:00'
SELECT T3.ProductName FROM Orders AS T1 INNER JOIN `Order Details` AS T2 ON T1.OrderID = T2.OrderID INNER JOIN Products AS T3 ON T2.ProductID = T3.ProductID WHERE T1.OrderDate LIKE '1998%' ORDER BY T3.UnitPrice + T1.Freight DESC LIMIT 5