Adventure Island is another well-known NES title, however, Adventure Island II is an even better sequel. The game took the original Adventure Island's gameplay and added new features such as an inventory system that let players bring along animal friends that the player saved or any weapons they found along the way.
Other changes and improvements this game made over the original included: shorter stages with no checkpoints, sound-based indicators of hidden egg locations, underwater stages, and vertical-scrolling stages.
Of course, the original Adventure Island would have been a great addition to the library as well. If we only got to choose one though, I'd vouch for Adventure Island II.