Show the id of the game platform with the most sales in region 2.
id of the game platform refers to game_platform_id; the most sales refers to max(num_sales); region 2 refers to region_id = '2'
SELECT T1.game_platform_id FROM ( SELECT T.game_platform_id, SUM(T.num_sales) FROM region_sales AS T WHERE T.region_id = 2 GROUP BY T.game_platform_id ORDER BY SUM(T.num_sales) DESC LIMIT 1 ) T1