Wonder Boy III: The Dragon's Trap
In my opinion, this wasn't just a great game of the time, it still stands as one of the best adventure / platform games ever. It feels just as good to play today as it did when it came out in the late 80s, and like Gaz_2_k, I still have plenty of time for it.
Responsive controls, great 8bit graphics, FM music (for owners of a Mark 3 system) and a simple but effective plot (secret rooms, items and cool puzzles) make this an all time classic.