Immortal Technique is definately my #1.
Get Revolutionary Vol.1 and Vol.2 and they'll be the only 'underground' hip-hop albums you'll ever need (almost). A couple of songs, specifically on Vol.2 really evoke emotion (which is hard in hip-hop). Dance with the Devil on Vol.1 is probably one of the most meaningful hip-hop songs ever written.
Swollen Members are also incredible.
EDIT: I'd throw in a vote for Dead Prez too, but they have been mentioned.
Get Revolutionary Vol.1 and Vol.2 and they'll be the only 'underground' hip-hop albums you'll ever need (almost). A couple of songs, specifically on Vol.2 really evoke emotion (which is hard in hip-hop). Dance with the Devil on Vol.1 is probably one of the most meaningful hip-hop songs ever written.
Swollen Members are also incredible.
EDIT: I'd throw in a vote for Dead Prez too, but they have been mentioned.