at the moment i would say its Snoop, but he is nowhere near the old days of Eminem.
Eminem or 50 Cent
I'm a big fan of Tha Blue Herb, a Japanese hip-hop artist with a really unique, calm flow unlike any I've heard.
Oh, or Astronautalis -- that guy can fucking spit when he wants to, and the music is way better than anything in most of the hip-hop game right now. That whole "post-rap" (God I hate that term) scene is actually really awesome.