I'm surprised no one has mentioned Bob Marley yet, not that I disagree with anyone's choices so far. For me personally, it's either Bob, Brad Nowell, or Jimi Hendrix
I'm with the OP when I say The Notorious B.I.G. I love Marley and Hendrix, but Biggies stuff was revolutionary in the rap genre and it is still some of the best rap out there, even today. He was about 10 years ahead of his time with his rap lyrics and style, and sort of made the shift into what the genre is today.