What exactly makes this the best 2D Mario game? People actually mean it's better than 3/World?
I haven't played a 2D Mario in a long time and this has been on my radar.
It's my second favourite 2d Mario, behind World, and ahead of 3. I find it really hard to quantify why, since these games are such pure gameplay experiences. It's based much more on World than anything else though, with new ideas thrown in. It has a really good level of challenge in the right places, and there's a specific challenge mode too that's really good.
The Luigi DLC is the toughest Mario game they've ever done as well, and that's included.