I think that most people consider Super Mario Bros. 3 to be the best of the Super Mario Bros. games. However, my favorite will always be Super Mario World. To be fair, Super Mario World was the first game I played in the series after the original Super Mario Bros. I wouldn't play the second or third games in the series until much later.
Super Mario World was included as the pack-in game when the Super Nintendo was released in 1991. No doubt this helped system sales in the early days. Super Mario World didn't offer a lot that was completely new from SMB 3 but the graphics and sound were improved considerably and the game just had a more polished feel to it. It also did have a few changes that affected gameplay such as the introduction of Yoshi and some pseudo-3D effects. To me it really represents the perfection of the SMB series and the side-scrolling platform game in general. As much as possible anyway...
Read more: https://www.megalextoria.com/wordpress/index.php/2017/08/22/super-mario-world/