Well I havent been at all interested in the rap scene for a long time now, but I can say without a doubt check out Anticon. Most importantly Sole, because he's like, amazing.
I would highly reccomend trying to find the anticon Comp album "Music for the Advancement of Hip hop"
www.anticon.com
I would also reccomend other semi popular underground artists like MF Doom, Virtuoso, et all
Basically if you want good rap you cant expect to hear it on mtv/radio/bet/whatever. Just like any other music