There are lots of great mcs who have the skill, but not the voice.
Then there are the rappers like lil'wayne....don't even get me started on his horrible voice.
In my opinion, Chuck D has/had the best voice in hip hop, lets face it. His voice was so powerful that u just had to listen.
Slick rick also had a great ORIGINAL voice, something we don't see everyday
Other great voices: GORETEX, Eminem(when he released infinite), Zaakir, Chali2na, Akil, Xzibit, Redman, Methodman, Gza, Inspectah Deck, ODB ofcourse, K-solo, Aesop Rock, Rakim, KRS-One, Big daddy kane, Melle Mel.........