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.