Galaxy for me, and it's not even close.
The central gimmick in Galaxy, the gravity mechanics, just gels perfectly with the most important thing in a Mario game to me: Jumping. Doing a long jump on a small planet and circling it entirely never got old. Add to that, the powerups, while time limited, almost all added to Mario's moves instead of replacing them. Highlight being the Ice Flower allowing you to skate on water and wall jump up waterfalls. The powerups that didn't were the lesser ones, like the Bee Suit or the Spring. They took away abilities.
In Odyssey, the main gimmick of the game is to not play as Mario. They give Mario a lot of moves, and then encourage playing as the more limited enemies over and over. You even fight bosses as not-Mario. Because everyone wanted to fight a giant bird as one of those lava things, right? The unfocused level design and all that padding didn't help either. Because when Oddyssey is good, it is amazing. But it's rarely that good. Mostly bland walking from point to point with nothing to jump on or over.
To be honest, for me, the question of best 3D Mario doesn't come down to Galaxy vs. Odyssey. It comes down to Galaxy vs. 3D World. The former has the most satisfying jumping in the series, an amazing soundtrack and creative levels. The latter has 4 player co-op, extremely sharp platforming thanks to locking Mario to moving in only 8 directions. And you can play as Peach. And Peach gets to use Fire Flowers. Fire Peach is easily my favorite look for Peach.