Many here seem to be interpreting vocabulary size as a signal for quality. When it comes to rap I completely disagree. Firstly, the repetition is rap's main ingredient. I read an article a while ago where researchers found that listening to a spoken phrase that is looped activates the same part of the brain as music, which helps explain this phenomenon.
Personally, if I want food for thought I read. Rap is not an intellectual pursuit. I've been perusing rappers on this list, and the top artists have not been good at all to my ears. It seems that the best rappers are in the middle, and being on either extreme is a negative signal.