Easy Galaxy vote. There's still so much more you can do with the setting and gameplay style. The gravity is one of my favorite mechanics in the series and makes all kinds of places feel unique. I love the variety and pacing of missions, with a scenic palate cleanser between miniature challenges. There's the vast, stunning atmosphere, the greater emotional depth, the backstory, all things particularly associated with… well, the first one, at least, and hopefully after such a long gap, the developers would want to create something that lives up. The art direction is timeless, and I always thought these virtues would only be further enhanced with an HD-first trequel. I think it says something that Galaxy 2 already exists and so many people still want another game in this style. (Also I'm still not over how I thought they called it the Wii U so they could make Super Mario Universe. There's my tragic backstory)
Now with that said, I could be down for a sequel to any of these games since I think they could all be bent to my tastes. I just think that outcome is less likely or would leave less of an impact on me. Like sure they could absolutely make Super Mario Odyssey Good for me if they fix, the Whole Thing You Do in the Game, but that might feel like a sequel in name only if it might as well be a new installment with new branding to draw people in.
I am honestly surprised that they never made a no-frills, same-scope sequel to 64, though. Just put together a sesquidozen little diorama worlds with a few collectibles sprinkled in, reuse the general progression systems… The game is still ubiquitous, with a lot of the criticisms being points that feel dated or have since been outdone, and they were even planning sequels at the time… Seems like it would've been easy money at any point in at least the past two decades since 64 DS? It's not my vote, but why not do that, you know?