Galaxy to me was like Breath of the Wild was to Zelda. It took a franchise which already had a masterpiece (Super Mario Bros 3, IMO), and then totally eviscerated it along with all of the Mario’s in between. The story, the spherical platforming, the freedom, and how polished it all felt at the same time as being ambitious and creative.
As much as I appreciate what Galaxy 2 and Odyssey did, they do not top this experience to me. I still love revisiting that first journey. IMO, Galaxy 2 actually improved on Galaxy 1, it’s got better levels; but I always see it as kind of an epilogue story or expansion, rather than the main show. But! IMO, Galaxy 2 is an essential play after Galaxy 1... kind of like DKC2 and 3 after 1 (Grrr SNES Classic!!!!).
Odyssey is such a great game that it’s hard to not want to praise it. but IMO, SMB3 is the clear series masterpiece, and SMG is the undisputed Magnum Opus. Especially if you consider SMG2 as the “Master Quest” of it.