Nas - Illmatic is the most essential rap release, in my eyes and in many others'. Song picks: Life's A B****, Halftime, It Ain't Hard To Tell
A Tribe Called Quest is peace-oriented and socially conscious. A lot of rap newbies love them. Get their first three albums. Songs: Excursions, Check the Rhime, Electric Relaxation, Can I Kick It?
Black Star is a group of two rappers, Mos Def and Talib Kweli, who came after Tribe but had a similar mission. They released one stellar album together, and a number of great albums separately. Songs: Black Star - Definition, Respiration; Mos Def - Mathematics; Talib Kweli - Get By, Shock Body
Kanye West: The College Dropout and My Beautiful Dark Twisted Fantasy are very, very different albums, but both are must listens. As are Late Registration and Graduation, albeit slightly less important. Songs: Through the Wire, Spaceship, Gone, Flashing Lights, Runaway
OutKast is another duo with a more decadent style, but they are one of the best rap groups ever, simply put. Songs: Rosa Parks, Spottieottiedopaliscious, B.O.B., actually get everything they've ever put out. Big Boi, one of the members, has a solo album that's equally as fantastic as his group's work.
my advice: get albums, not songs.
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Black Turtle shoots the lights out