Freezeflame or Bouy Base. Oh and Toy Time was pretty pretty neat. Battlerock and Sweet Sweet were also great. Melty Molten was frigging fabulous and Gusty Garden was simply kick-ass. And let us not forget about Rolling Gizmo and Beach Bowl, either.
And I still feel like I'm forgetting something. There are so many great galaxies in SMG, that it amazes me that Miyamoto & pals actually have enough creative juice left for a sequel...