Donny Hathaway, both the artist and the album, are criminally underrated. That's the reason I have this recommendation categorized as a hidden gem, because it is absolutely a classic soul album.
Donny Hathaway excels at songwriting, as well as his voice and delivery, all of which are on display throughout the album.
The first song on the album, "Giving Up" is downright amazing. Not only does it feature a peak form Donny Hathaway, but it's one of the most beautifully composed songs I've ever heard. It strikes the perfect balance between complex and smooth. Play it for a music theory enthusiast, and they'll delight in the chord progressions, key changes and the expert use of instrumentation as accents throughout the song. Play it for a regular music lover, and it will be impossible not to be struck by the pure beauty and emotion that it exudes.
Donny Hathaway, the album, is an under appreciated masterpiece, and Donny Hathaway, the artist, is one of the great musicians of our time, who never seems to get his proper adulation.