I would probably say Super Mario Galaxy 2, if we're talking the mainline Mario platformers and ignoring subseries like RPGs are platformers from other Mario franchise characters like Donkey Kong and Wario. If we included those, I think I'd have to go with Donkey Kong Country 2.
I originally looked at Galaxy 2 as a kind of lazy expansion of the prior game, much like how I saw New Super Mario Bros Wii to be the same for its prior title. However, both games here actually feel like more evolved and more "full" experiences, especially Galaxy 2 when compared to the first.
Galaxy 2 kind of makes the first game look like a trial title to test concepts and ideas, which is
remarkable to actually say as the original Galaxy is considered one of the best games on its platform, of that generation, of Nintendo's first party work, and one of the best games ever made.
Here is hoping Odyssey hits the same highs, even if it's going more to the sandbox of 64 and Sunshine. I hope for the level of variety that the Galaxy games offered and not a random collectafest like Donkey Kong 64.