I only started playing Mario Odyssey yesterday, so it's too early to put it on the list (though it's certainly going to go high on the list, I'm enjoying it immensely).
As for my favourites (not counting spin-off games like Mario Kart, etc, and on the fence on how to count Mario Maker, so leaving that one off as well), I go back and forth on the order of my top three Mario titles, simply because I feel they offer so vastly different experiences that it's hard to compare them.
Super Mario Bros 3, Super Mario World, Super Mario 64
Depending on my mood any of these three could go on any of the top three spots. SMB3 is king of the power ups (and secrets), took the original and just went to town. SMW scaled down the power-ups, but introduced movements, depth and Yoshi, plus it's immense in scale. SM64 just did such a great balance between world design and giving you the freedom to roam within each of them (one of my issues with the Galaxy games was how forced the linearity was).
Number 4 on my list is Super Mario 3D World. I know it isn't the only core title to have simultaneous play, but I guess I especially love that you can do this in a 3D Mario now. I've had a lot of fun with this one.
I do think Odyssey is likely to become my favourite because it does seem like such a great tribute to the history of Mario and give you a taste of what came before while giving you new things as well. But I will not name it a favourite yet. I have to properly play it first.