RJD2 Rick Rubin
I actually came back to this thread to modify my post and add these two.
Also add Mike Shinoda, just for his ability to mix different genres so well and make it sound brilliant. He is still new though compared to the others mentioned./
Yeah, I wouldn't call Shinoda the best ever. Great, but not quite there.
My money? Primo is probably who I'd pick. It'd be close though. Contenders are Erick, Dre, Large Pro, Rick Rubin, JMJ. It's a short list.
RJD2 ain't on there. Again, good, but nowhere near the best. I wouldn't put 9th Wonder, Illmind, M-Phasez or Black Milk there for the same reason.