Super Mario Land 2: 6 Golden Coins (Game Boy, 1992)

The trend for major developers making games on the Game Boy was to simply make watered-down NES or SNES ports.  Capcom, while being one of the best third-party developers for Nintendo, were certainly guilty of this by their Mega Man and Ducktales games for the handheld.  The first Super Mario Land, while being in every right a classic Game Boy game, has not aged well due to its high difficulty and tiny character sprites.  Super Mario Land 2: 6 Golden Coins (1992) breaks this mold by creating a handheld Mario game that is at a level of quality equivalent to a console release.
Atypical of Mario game's plots, this one is not a quest to save a princess, but to reclaim Mario's castle from the greedy and despicable Wario.  He has taken the six golden coins needed to open the gate and given them to his minions who have scattered across the land.  Mario must travel to six different zones on the map (a la Super Mario World) to retrieve them.  This gives the game that same non-linear feel of the other recent Mario games.  The worlds each have a different feel and play mechanics to them, which keeps everything fresh.  One zone is a giant tree.  Another has Mario shrunken down so that an ordinary house is humongous and its everyday appliances are hazards.  My favorite is the Mario Zone, which takes place inside of a giant, clockwork Mario.
The game features similar power-ups to Super Mario Bros. 3, but not nearly the abundance.  There is, of course, the fire flower, but new and only appearing in this game is the carrot, which turns Mario into bunny Mario.  If you hold the jump button in mid-air, Mario will flap the ears on his cap, allowing him to safely float down.  Remember, the Mario games forgo realism in favor of a more fairy tale feeling.  These two power-ups are handled well, with certain levels being made easier by using either of them, and some making them crucial.
One thing that is noteworthy about this game is the presentation.  My credo is that a game should not be judged by its graphics alone, but I always expect that a game take full advantage of its platform's capabilities and use an art style that is visually appealing, and Super Mario Land 2 certainly does this.  The original Game Boy was only capable of black, white, and a few shades in between.  To get around this the game features a cartoon style that was optimal for the system (see the also excellent Gargoyle's Quest).  Sprites are well-drawn and animated, and the music is also composed to take full advantage of the 8-bit music processor and is memorable.
The Game Boy was not the most technically impressive system to make games for (it was, in fact, the least powerful handheld on the market at the time, but the most popular due to its excellent game library and long-lasting battery life).  To make an ambitious game on it was incredibly risky, but Nintendo knew what they were doing for their little gaming brick, and crafted a game which, while not perfect, has aged far better than most of its contemporaries.  There is an odd kind of timelessness to all of the best Mario games, which I think mainly comes from the fun they generate.