There are over 40+ Galaxy's in Super Mario Galaxy. And out of those Galaxy's, which one do you think should have been used more? I'm talking more about those bonus Galaxy's with only one star.
I think the Matter Splatter Galaxy could of been used a bit more (Even if there is one more level that used it.) I loved the idea of that level, how the floors and walls disappear and reappear and that little drip that makes the floors and walls shink and expand. My second choice would be Bouy Base. (I forgot the name)