Rescue the Princess.

KOEI is famous in NESMania for making really long games that most viewers don't wanna see them, but coming from a player this one is actually a very good game. The plot is more less that you need to restore the honor of your family in Portugal and become a famous navigant/explorer of the seas and lands around the world. There is some quests and special request from the King and the more you complete the Higher your title will be, which makes this interesting and gives you a feel of progress, also you can fight other ships and pirates too. Make money by selling and buying stuff from different parts of the world. Surely this game is unique for the NES and huge point in it's favor is that the music was very enjoyable.