Designer and data scientist Matt Daniels charted this neat comparison of vocabularies in hip hop. At the bottom of the chart you can see DMX with only 3,214 unique words. Outkast has more unique words in their lyrics than Shakespeare, but everyone is way behind Aesop Rock, who rules with 7,392 words.
Aesop Rock is even ahead of Herman Melville's Moby Dick. Here's how Matt put it together:
35,000 words covers 3-5 studio albums and EPs. I included mixtapes if the artist was
of the 35,000 words [...]
I used the first 5,000 words for 7 of Shakespeare's works: Hamlet, Romeo and Juliet, Othello, Macbeth, As You Like It, Winter's Tale, and Troilus and Cressida. For Melville, I used the first 35,000 words of Moby Dick.
All lyrics are provided by Rap Genius, but are only current to 2012. My lack of recent data prevented me from using quite a few current artists.
This data viz uses code by Amelia Bellamy-Royds's in this jsfiddle.
Make sure to visit his interactive chart here.