This game took Super Mario Bros into intriguing new territory, and did so very successfully.
Super Mario Bros 2's base concept is still in platforming, but rather than jumping on enemies to eliminate them, this time jumping on enemies can only be used for transportation. It adds an intriguing new level of depth to the game, and while it was removed in Super Mario Bros 3, I like this style of gameplay quite a bit. The level design is also very non-linear at times, and I get a greater feeling of exploration from this game than I do from the original Super Mario Bros. Graphics-wise, this game is also excellent for an 8-bit machine - everything is sleek and colorful, and the music is every bit as good you'd expect from a Mario title.
This 1988 2-D platformer is one of my absolute favorite NES titles, and although the second half of the game occasionally gets too tough (for me, at least - I'm horrible at platforming games), this is a near-flawless NES adventure. This is an under-appreciated game for sure, and anyone who enjoys platformers that challenge your fingers and your brains owes it to themselves to play this awesome NES title.