Classic Mario that has a lot more to offer than the first.
The Levels are pretty neat. Most have great scenery and cool little mechanics. Some however aren't so fantastic. Every world end with the same style flying ship and same style boss. This is still forgivable since this is only the 3rd Mario game. The world are also really nice and vary widely.
Where I think the true strength of the game lies is the new power ups. The Hammer suit lets you throw Hammers sort of like Fire power but unique and different. The Tanooki suit lets you turn into a stone statue and crush enemies. This suit not only looks cool but has some usefulness. Remember those dreaded swimming levels? Well now the frog suit makes those levels a breeze. But the Frog suit is difficult to maneuver out of water.
Those aside Super Mario 3 also lets you keep items to use on the map. Some can even help you skip levels.
Mario suffers from some slight repetition but makes up for in classic game play and charming levels.