What's essential will depend on every individual, but these are some albums that are very informative to my taste:
* Exmilitary - Death Grips
* Led Zeppelin IV - Led Zeppelin
* Plastic Surgery Disasters / In God We Trust, Inc. - Dead Kennedys
* Black on Both Sides - Mos Def
* Rage Against the Machine - Rage Against the Machine
* The Clown - Charles Mingus
* If I Had a Hammer - Pete Seeger
* Master of Reality - Black Sabbath
I could probably list a hundred more, but that's the skinny.