There was an article that came out some time ago looking at the size of rappers vocabularies, and a suspicion of mine about DOOM was confirmed: that he used more unique words than almost everyone else. https://pudding.cool/projects/vocabulary/index.html

Biggie didn't qualify, but my thesis from the article was that the size of vocabulary was irrelevant. Biggie and DOOM for me are the hardest rappers to replicate, not because of how many unique words they use, but the way they reinvent the language of rap while working within it.

Biggie will take a word like "automatic" and turn it into "auto/mat/ic/" which completely changes what you expect from the rhyme in a song, and what comes after it. He'll say normal rap things, in different ways that's refreshing without overtly trying to be "intelligent"

One of my favorites is from "Victory" where he says:

Francis, M to the iz-H, phenomenal/
Gun rest under the vest, by the abdominal/

It's fun because all rappers have talked about keeping a gun tucked, but how many times do you see it phrased as resting by the abdominal?

It's impossible to replicate beyond doing his cadence because it's a creative project in understanding the language of the music well enough to toy with, rearrange, and refresh them whenever he pleased. Sometimes it was chopping up words. Other times, injecting new metaphors.

Basically, you can give a great rapper ten words, and they'll make the gravity of them felt. Biggie can take ten words and change the reference points and expectations of them so that they feel like a hundred.