Super Mario Bros. is a near-perfect recreation of the levels in the original Super Mario Bros., though there are a few inaccuracies. It is the first mappack created for the game, and World 1-1 is seen in most pre-release screenshots.
Mari0: SE Edit
In Mari0: Special Edition, you cannot view this mappack normally. This is because its settings.txt is missing from its folder, but you can fix this by adding one via your mappack folder or through modding. Additionally, the Minus World is missing. This can be remedied by editing 7-2. Hook the pipe at the end to Main, then, in your mappack folder, rename it to "M-1.txt".