Super Mario Galaxy 2/World S
World S is the legendary world that had been said couple of times before beating Bowser. Once you had beaten Bowser in the Galaxy Generator, the game will say that a World S had opened up.
World S will have 6 of the hardest level of the game. While it is also include the Grandemaster Galaxy, that galaxy ordinary star and prankster comet star will appear only on some certain condition.
To be able to take all the galaxies (except for the last one that requiring also star bits), you will need to gather 110 stars of the game (out of the all 120 stars of the game). There's no forks in this world, so you will have to take every galaxy to advance.
The save title on this world would be: "World S: Here we go!".
(Note: the guide to this galaxy contains only the gold stars levels)