The Nintendo Entertainment System has some really awesome features in terms of his electronic design, and one of the most important is the capability to make music with the main processor (2a03, a modified z80) and also with external chips inside the cartridges.
The "Expansion Audio" enable the composer to use more squarewaves, samples and also some FM synthesis. And because of that, this kind of games have the most amazing soundtracks ever.
Akumajou Densetsu (Konami VRC6)
Batman: Return of the Joker (Sunsoft FME-7)
Gimmick! (Sunsoft 5B)
Lagrange Point (Konami VRC7)
This four games have the best NES soundtracks in terms of hardware capability and music composition.
This is the most badass song from the NES era ;)
These soundtracks are amazing and still hold up. My favourites are definitely Castlevania 3 (Famicom Version) and Gimmick
You missed Ducktales. The moon level is one of the finest compositions for the Nes.