Unless I missed it, I'm surprised nobody mentioned Isaac Hayes. What he contributed during the period between the late 60's and mid 70's was unprecedented musically. I strongly encourage anyone who loves sample based Hip Hop to explore his discography from that time frame.
Hot Buttered Soul, Joy, The Isaac Hayes Movement, To Be Continued, Black Moses, Shaft, Three Tough Guys (I only recently became aware that this brought us Mind's Playin Tricks by GB), and Truck Turner are all must haves.
edit post: There's artists with better discographies than Isaac Hayes, but in the context of musical geniuses, only a few artists come to mind. When I say that, I'm talking about people who encompass all the different elements of being a musician and artist. Hayes not only was the main vocalist of his work, he arranged the projects, created the projects, played multiple instruments, etc.. The only artists I can think of in a similar vein would be your Princes, Curtis Mayfields, Quincy Jones, etc.. Can't say the same for many singers or rappers who had great producers behind them like Al Green(Willie Mitchell), or Outkast (Organized Noize)
Justice league