While I find SMB1 to be pretty good and Lost Levels/USA to be the standard messy sequel, SMB3 feels like the proper upgrade to the original SMB1 in everything possible.
More upgrades for Mario, tighter level design, one of the most beautiful soundtracks for a game of this time, a bunch of minigames and a ton of freedom for the player to do whatever they want to do.
Maybe my only grasp with this one is the obscene amount of scrollers in this bad boy.... doesn't help that every final stage in the world is pretty much one, too.
The difficulty in some spots can be borderline trial and error, too. Which is fine, the game hands you extra lives like a madman, but I'm not too fond of it. Possibly these two things are what push this game a bit back for me at least.
Still, this game was my absolute childhood, alongside SMW! It's definitely a gold standard when it comes to difficult platform games.

Shigeru Miyamoto's masterpiece. This game took everything from the first title, and a few things from its peculiar sequel and just turned them up to 11 here. This was a time when Nintendo's games excelled at showing, not telling, you what to do. Every level would either introduce some new design, or a unique play on one of your powerups, or subvert your expectations like the first time a regular looking block grows legs and jumps at you!

The fact that it managed to keep this level of creativity going all the way through the end is nothing short of spectacular. When you combine the crisp sprites, colorful visuals, and wide-ranging soundtrack with that level of creative gameplay, you get nothing short of legend.