To go back to the original question of the thread, SMB3 or SMW?
I'm old enough to remember when this game first came out, and it was a HUGE deal. I was blown away by it at the time, and to this day it remains my favorite game ever to hit the NES (as well as my favorite Mario game so far).
The game had everything: It was sleek and stylish looking for the time on an 8-bit system, the music stuck with you, it had a plethora of power-ups and items (Hammer Bro. Suit FTW!), the Koopalings (BTW it was awesome that they came back in NSMBW), appreciable challenge with a fair learning curve as you went along (not too hard, not too easy); it's one of the longest games on the NES, and you could make it as long or as short as you pleased by warping. One thing that I absolutely loved about this game but hasn't been seen nearly as much in Mario games since are the world themes. We have Grass, Desert, Water, Giant, Sky, Ice, Pipe, and Dark Lands (not even NSMB or NSMBW have seen such diversity)--and the stages within those worlds did such a marvelous job of keeping to all eight of those themes but still having diversity within themselves at the same time. This whole game is nothing short of a masterpiece of level design. (This is part of my problem with games like Super Mario 64: instead of a multitude of diverse stages, you're given only a few that you have to redo over and over--blagh!)
Don't get me wrong, I still
love SMW. Super Mario World and Yoshi's Island both are very high up on my list (second only to SMB3, in fact) and also exhibited marvelous stage design and imaginative world detail. Heck, I even liked SMB1 and SMB2 as well, even if not quite as much as my top 3 of SMB3, SMW, and SMW2:YI.