Mario vs. Donkey Kong 2: March of the Minis
I'm still cruising on gold stars, but it may come to an end soon. World 6, Toadstool Castle, is rife with dangerous situations that require constantly switching attention back and forth to ensure the survival of the mini-Marios. While one is riding a life raft over a pool of water on the right side of the screen, the one on the left has to hit the colored switch to keep it protected from a piranha plant's fire breath, which in turn removes the barrier that had previously kept little guy #3 from falling onto a bed of spikes.
Right, each world has a theme for its nine stages + DK boss battle. The first two or three were distinguished almost purely by aesthetics, but as they progress, world-specific elements make for some interesting encounters. World 4 is called Magnet Mania, wherein the mini-Marios can walk on walls and ceilings (very cool). World 5 is the Lava Dome, and there is basically fire everywhere.
Though not at the same pace as the beginning of the game, new puzzle pieces are still finding their way into the mix, keeping the new stages fresh. And, as I implied in my discussion of world 6, the challenge is really ramping up.
Progress: Stage 6-4