My favorite Mario game second only to Mario 64. I loved the creativity, and the addition of Fludd (though I don't really LIKE Fludd as a character) made the whole thing feel so fresh and interesting. I also loved the setting, and the feeling of a new world. It seems like today Nintendo wouldn't do something like that and it would just be populated by Toads (I'm looking at you, Color Splash), but then along comes Odyssey to prove me wrong. In some ways it feels like Odyssey is the spiritual successor to Sunshine, so I can't wait.
Also if they would ever make my dreams come true and put GameCube titles on a Switch VC, I would buy this game again in a split second. No question.
It definitely has some flaws, but it has the best and most fun mobility in a 3D Mario game, and, therefore, in a 3D platforming game. Odyssey looks to be taking a page from Sunshine in terms of mobility. Fingers crossed!
Also, I DON'T agree with the consensus that the FLUDD-less parts were the best sections. I really liked the extra options that FLUDD gave the player. Slipping and sliding was amazing.
My main problem with this game was the extreme lack of polish, but I still enjoyed parts of it. The FLUDD mechanics worked pretty well and added to the game, though I think it was a mistake to make it a character. I understand why they did though, since Mario doesn't speak.