Greydon Square. He focuses on atheist and science related topics and there aren't many rappers who do this, at least not many talented rappers, as he is. Since almost all mainstream rappers are theists of some sort, we are in desperate need of atheist rappers because there's a treasure trove of untapped subject matter there. Hip Hop is also a great medium for getting the atheist message out. Imagine an atheist debate that is entirely done in the form of a rap battle. I'm sure its been done, but probably not how I envision it. The ideal atheist vs theist rap battle would go like this. The theist would open with his argument in rhyme form, then he'd send the transcript to the atheist, who'd craft his response and send the transcript to the theist, who'd then respond and do the same. That way they'd each have time to directly respond to the claims the other made. It wouldn't be able to be live though, but it could be prerecorded and put together. This definitely needs to be done.
Anyway, check out some of his work, promote his music, buy his albums, and if he's performing in a town near you, go see him live.