Super Mario Bros. 3.
Easy pick, it's the one that feels the most fluid to me; World is too floaty and slippery for my tastes and the original too choppy. The 'New' series is just uninteresting really, I mean the games are fine but they're just 'there'.
For a new one, which I don't particularly need per se, I'd say they only just have to come up with something unique, also in gameplay. I like a presentation like Mario Maker 2's 3D World style, a bit more fleshed out for example.