Without a shadow of a doubt, I consider this the greatest Mario game in the series. With its rich colors, catchy music, and the addition of Yoshi; this game sucked days and weeks from my life.
You won't grow bored of the wide variety of secrets in the game. Thanks to the dozens of levels in the game, you'll be spending a lot of time going back and redoing parts just to find a new passageway or level.
Although the difficulty did go down (that is if you compare it to Super Mario 3), it has become a lot more opened.