THERE ARE TWO "PERIODS" OF RAP - BEFORE AND AFTER THE BLUEPRINT.
Essentials - start here
Jay Z - The Blueprint (Modern, NY)
Nas - Illmatic (Vintage, NY / "Dust")
Outkast - Stankonia (Modern, Southern)
Kendrick Lamar - Section.80 (Modern, West Coast + Conscious)
Kanye - Late Registration (Modern, Soul-Sampling + Orchestral-arrangements + Conscious)
Lil B - God's Father (Modern, Based)
Deeper We Go - once you have the above covered
Kendrick Lamar - good kid, m.A.A.d city (Modern, West Coast + Conscious)
Big Boi - Sir Lucious Left Foot (Modern, Southern)
Big Krit - Live From the Underground (Modern, Southern)
Kanye West - College Dropout (Modern, Soul-Sampling + Conscious)
Nas - Life Is Good (Modern, NY)
Kanye / Jay Z - Watch The Throne (Modern, Braggadocious + Conscious)
Chief Keef - Finally Rich (Modern, Drill Scene)
That should be enough of a background to give you perspective of the genre. Just remember, The Blueprint is arguably the most important hip hop record from a genre-history perspective.