Super Paper Mario, if talking about the franchise.
Out of the ones on the poll, it's actually pretty close between Galaxy, Odyssey and 64 (though perhaps nostalgia is the only thing keeping 64 in the running for me). World is great and probably the best 2D mainline Mario if the Maker games aren't counted, but I overall find more hype in the 3D games than the 2D ones (again, excluding the Maker series, and Super Paper Mario but I like it moreso for the story than the 2D platforming)
I think the Maker games, if counted as mainline, would easily claim the crown when talking mainline games only just because of their online experience and the ability to create the levels of your dreams and play those of others, there's nothing quite like that.
What I find strange about this poll is that Super Mario World is the odd duck out of these, as it's the only 2D linear classic Mario platformer, while the poll primarily has 3D Mario entries and also leaves out Super Mario 3D Land and Super Mario 3D World.
I think the poll maker just decided to look at the game titles rather than their actual relations to each other.
Within the limited and arbitrary poll (which I think just picked games that were commonly perceived to be the best) I picked Super Mario Odyssey. It did away with the tedious lives system for starters (though I think it should be sped up even more to the point respawning is nearly instant as it is in Rayman Legends) which streamlined the gameplay a lot. It did what Galaxy failed at; where Galaxy kept trying to tread the line between linear platforming and the joy of exploration and discovery but never really committed to it, Odyssey was a fine example of where Power Moons were very well mixed between discovery and linear platforming (as linear levels are still there but in doors, warp pipes, etc.) The goal-based mission format just seems archaic these days when a lot of them is just revisiting a level that's not much altered and doing more repetitive tasks. Also, the controls are great and responsive, the animations are top-notch, all levels are at least good in their own way, and the purple coins never got too cryptic. The soundtrack was best when no music was playing by the way; not a knock on the soundtrack but that ambience works so much better. It's a real shame Nintendo doesn't let you turn off the music in this game because people complained about how grating Mario Kart 8's soundtrack can get and I can imagine this one can get grating at times (looking at you, Fossil Falls).
But anyway, Odyssey also has the costumes which are amazing and a first for the series and I think it at least preceded the ongoing glut of wardrobe changes we see in Mario & Sonic and in Mario Kart Tour. Here's hoping the costumes from Mario Party 2 make a comeback...