Yoshi's Island DS
It's evident from the position of things on the map menu that there are five worlds in Yoshi's Island DS - which doesn't seem like much (each world having eight stages), but assuming there is some crazy Raphael Raven nonsense in the last world, this portable little beauty will have accomplished even more than the original game did.
My upbringing on the SNES still makes me feel a bit off playing platformers on a DS, but that feeling is very subdued here. Not only are some levels startingly challenging, but Artoon has really taken advantage of the system: while there are already a lot of games working wonders with the touch-screen, and a lot of games using a second screen for secondary purposes, Yoshi's Island DS is the first I've seen that actually extends the action across both screens. This vertical panorama is super-effective in creating a refreshing platform experience.
Baby DK can climb vines and dash into objects, breaking them with ease. Baby Wario has a magnet, which attracts metal objects and coins to him when positioned correctly (this seems gimmicky but enables a lot of great puzzles). And it turns out each baby also fires eggs a little differently - where Mario's will bounce off walls, Peach's will break on impact, and DK's actually have a blast radius. Still no word on Baby Bowser.
Progress: Stage 4-2