It's so conventional to talk about Galaxy 1 as the generation's revolutionary moment at the point of impact—and Galaxy 2 as the perfect, incremental refinement that finished Galaxy 1's sentences—that I think it's easy to lose sight of just how much about Galaxy 2 was equally novel. It arguably dialled back a lot of SMG1's gravity-related innovations—it was less exclusively focused on being about the Little Prince planetoid concept—but that just made it a summation of what the entire 3D series brought to the table by that point in the series' design history.
I will always, always be grateful for Galaxy 2 for introducing, with Grandmaster Galaxy and The Perfect Run, the idea that 3D Mario (and later, Captain Toad) should conclude with an unforgettable crescendo, a "final exam" level that brings everything you've learnt up to that point to a satisfying, chunky finish. This is the main thing I look forward to with every 3D Mario game now, and it was with Galaxy 2 that it began.
Also, everyone knows Gusty Garden as SMG1's towering instant classic in the repertoire of Mario music—but we all too easily overlook its stupendous younger sibling, the soaring melody of Cloudy Court:
I cannot wait to replay this game. I haven't revisited it fully since 2010, as my copy is with a family member and my Wii and Wii U are not on hand, and I've been longing to experience it all the way through again. After Galaxy 1, one could say, there are so many ideas here bursting at the seams that of course we all want more. After Galaxy 2, one said, where can you even go from here?