You could have called this thread "3 vs World" and no one would have argued with you.

melee

Super Mario Bros 2 USA.

I haven't played Lost Levels, 3, or World.

It's Super Mario World for me.

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.

x 1,000,000

Super Mario RPG on the snes is probably the best rpg the SNES had, even though it had plenty good ones.
Its a shame the direction they have taken Paper Mario these last few games in the series.
Honestly if they cant be bothered to make Paper Mario a proper RPG, they should work on getting a new Super Mario RPG game out (that can these replace that itch for a rpg mario game).

So if we're not counting the Galaxy series, then SMB World/SMB 3 is by far my favorite. Yes I picked both because I hold them both on the same tier of enjoyment, personally.

Super Mario world is probably the best game, but 3 is probably my favorite from a nostalgic perspective. Super Mario Bros is my most played for sure, trying to beat it in under 5 minutes. Got so good at it in College that I could be drunk, talking to people at a party at my place while speed running it and not die.

The original one, followed by World

Super Mario World with Yoshi's Island as a far second and SMB3 as a distant third.