Personally, for me, it has to be Slug without a shadow of a doubt. Aesop Rock's flow is pretty impressive, but you can't understand half of what he's saying sometimes. Slug is insanely smart, and his songs are so emotional and they show just how great hip-hop can be, and just how beautiful lyrics can be.
But that's just my opinion. What about you? Let me know in the comment section, as always.