Seriously, no one has mentioned Jurassic Five? They're the most talented rap vocalists on the planet. Take this song, Jurass Finish First:
http://www.youtube.com/watch?v=0QmnEEteDig
Listen to that first verse sung by Charli 2na. I'll post the lyrics here:
Yo, because of cash in the purse, guns blast in the hearse
A vast universe when the last is the first
The past been a curse, I need some asprin to nurse
It's your casket in earth, or my ass when it hurts
A passionate burst of some last-minute work
First the human bodies are living last in this Earth
Puffing grass when it works, a bastard at birth
But at last planet Earth, 5 Jurass finish first
Lyrically it's nothing exceptional, but the form is spectacular. I'll point out the alliteration for you.
Yo, because of cash in the purse, guns blast in the hearse
A vast universe when the last is the first
The past been a curse, I need some asprin to nurse
It's your casket in earth, or my ass when it hurts
A passionate burst of some last-minute work
First the human bodies are living last in this Earth
Puffing grass when it works, a bastard at birth
But at last planet Earth, 5 Jurass finish first
For almost every line they've managed to keep the beat, vocally, using æs (phonetically "ass") and ɜr (phonetically "ur") over and over again. It's impressive vocally, and it's impressive in terms of writing.
They may not be saying much, but they're saying it well. Better than anyone else out there, hands down.