Show the id of game platform which makes the most sales in Japan.
id of game platform refers to game_platform_id; the most sales refers to max(num_sales); in Japan refers to region_name = 'Japan'
SELECT T.game_platform_id FROM ( SELECT T2.game_platform_id, MAX(T2.num_sales) FROM region AS T1 INNER JOIN region_sales AS T2 ON T1.id = T2.region_id WHERE T1.region_name = 'Japan' ) t