Most notable would be G-Rap especially seeing how it works to his advantage. Rza, Ice T to a lesser extent.
The one with the most severe I would think would be Drag-On.
You can't hear it in his raps but I remember him doing interviews telling the host how it's remarkable how he can even rap due to having such a bad stutter in his every day life. I always fukked with him partly for being able to over come that.