I don't have much of a number one Mario game but Mario Kart and Mario Party remain two of my favorite series, and I also enjoy tennis, Strikers, baseball, Mario Sports Mix, golf a lot. Super Mario Odyssey is probably my favorite Mario game however. Just love roaming as Mario, hope Nintendo can somehow pull off an open world Mario; that'll be delicious, as I highly enjoyed BOTW and Horizon Zero Dawn and like the more open aspect of Odyssey despite that not being open world). I dig the genre, mmmhmm. Maybe sometime in the future, I just want an expansion to Odyssey, since there are some very lacking kingdoms like Ruined Kingdom and Mushroom Kingdom (which looks pretty depressing for a Mushroom Kingdom quite frankly, especially when you see it isolated from literally every other kingdom on the planet).
My absolute least favorite Mario game has to be Mario Kart Tour. I've said my piece on it, but recap: it's the only Mario game I deem harmful to play for anyone, parental advisory needed. It's the only Mario game that I know preys on its playerbase via exploiting people with ADHD, autism, and children through encouraging gambling. I've played it myself. I absolutely abhorred the gameplay, chalking it up to be Mario Kart but worse in nearly every aspect due to the messy slipshod controls, the sluggish speed, the boring as hell track designs that accommodate for the snoozefest auto acceleratre/auto steer/auto trick.
I don't see how any other Mario game's worse than a game that's not just a bad game but a complete hazard. I'd rather kids play Mario is Missing or Mario's Early Years Preschool Fun or Hotel Mario.