In Yoshi’s Island, the edges of the map screen are always obscured by scrolls of parchment. Removing the layer containing them reveals that the map takes up the full width of the screen. Among the things not visible during normal gameplay is a hidden monkey sitting on a rock at the far right edge of World 3.
If you could only have one Nintendo game, which game would it be?
But in all seriousness, if you mean 1st party Nintendo games, I would go with Yoshi’s Island. I think it might be the most polished, well-designed platformer of its era. I absolutely adore the atmosphere of the cave levels.