I have to say battlerock or Bouy Base. Battlerock was just epic. although there really was no galaxy i didnt like. I can say though, even though Toy time was incredible, that stupid spring mario ruined it. The controls just werent responsive or polished enough.

oh almost forgot, sweet sweet galaxy., loved it

I don't remember the name of the galaxy, but the one with the cute Honey Bee!

Toy Time..I hope they really expand it this time. Also, Mario Galaxy needs some sort of landscape to just run around but that's just wishful thinking.

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...