World 7 (New Super Mario Bros. Wii)
From the Super Mario Wiki
World 7 (also known as the Cloud World) is the seventh world in the game New Super Mario Bros. Wii. The area is sky themed and contains many clouds and high cliffs. It appears to take place above World 1, due to its mushroom-like hills. However, it does contain several unique enemies in its stages, and is highly abundant in items (mostly Ice Flowers, Fire Flowers, and Propeller Mushrooms). Ludwig von Koopa is the boss of the world, and he is fought in World 7- and World 7-. The Enemy Course in this area contains a single Lakitu that throws Spinies on a stage with Jump Blocks. There are also various Toad Houses, including two yellow ones that contain Stars. It has nine levels and a total of eleven exits.