Best and worst examples I can think of off the top of my head:
Open Mike Eagle - 10 out of 10 for a rapper. Can keep a tune. Always raps to the original melody. Live performances sound exactly like the record, or are better.
Action Bronson - 0 out of 10. Raps with a flat melody. Can't even say the hook in tune, let alone sing. Sounds like he's going to have a stemi at any moment.
So Mos Def, who has done this a couple of times. I'd give him 8/10
And then
Kid Cudi, who most of the time he sings I'd say is 9/10 to 10/10. He can sing with that R&B like sound which is pretty nice during breakdowns or chorus.
Don't really have any one I listen to that is all that bad. I mean maybe Kendrick but that's about it.