I was going to pose this question in Arce/Bill Anvil's thread, but I think it deserved its own GetBig post.
Now, I don't want to talk about background music, nice beats, production values..I just want to talk pure, lyrical ability.
When music, beats, and hype are factored out, the list shrinks considerably.
I'll start:
Rakim
Eminem
Kool Moe Dee (not widely heard of, but rapping ability is awesome).