Havoc is probably should be the number one rapper who has proven to be qualified in both fields (rapping and producing), and like others said above Erick Sermon as well.

While I will say it is others who are better producers than rappers but some people on that list, Havoc's output of produced joints and spat some dope verses than most but usually gets overlooked cause of Prodigy but he usually goes toe to toe lyrically with Prodigy.