Great lists you guys. I'll add a few that haven't been mentioned that are essential:
Pete Rock & C.L. Smooth - Mecca and the Soul Brother
Brand Nubian - One For All
Diamond D - Stunts, Blunts & Hip-Hop
Jungle Brothers - Straight Out the Jungle
Slick Rick - Great Adventures of Slick Rick
Redman - Whut? Thee Album
Eric B. & Rakim - Paid In Full
Gang Starr - Daily Operation
NWA - Straight Outta Compton
Run DMC - S/T
EPMD - Strictly Business
Showbiz and A.G. - Goodfellas
Digital Underground - Sex Packets
Beastie Boys - Paul's Boutique
Showbiz and A.G. - Runaway Slave
BDP - By All Means Necessary
X-Clan - To the East, Blackwards
De La Soul - Stakes Is High
Park-Like Setting - School Day 2, Garbage Day 4
Artifacts - Between a Rock & a Hard Place
Main Source - Breaking Atoms
The Roots - Illadelph Halflife
The Roots - Do You Want More ?!
Jeru the Damaja - The Sun Rises In The East
O.C. - Word... Life
John Smith - John Smith's Blunderbus or, "in transit"
Ghostface Killah - Ironman
All of these albums are Truly essential.