The question as to who is the most influential person in the history of hip-hop is a big one, but there's only one true answer: 19th century British Romantic poet William Wordsworth.
Hear me out.
Hip-hop has always been an artform based around the individual. Style, backstory, and individuality are valued almost as highly as artistic ability. The rapper is his own best subject. Of course, an MC is still allowed to investigate the outside world, but rap lyrics inevitably tend back towards the rapper himself.
I would claim that you can trace this tendency back to good ol' William Wordsworth.
Literary critic Harold Bloom writes the following about Wordsworth: "Before Wordsworth, poetry had a subject. After Wordsworth, its prevalent subject was the poet's own subjectivity." Put another way, Wordsworth made it cool to talk about yourself in your writing. This innovation stuck, and you'd be hard-pressed to find a poet writing today who doesn't follow in Wordsworth's footsteps in one way or another.
You'd also be hard pressed to find a rapper who doesn't do the same.
Wordsworth's innovation to put the artist at the center of the art affected just about every medium, from literature to painting. Hip-hop added another innovation: it's a competitive artform. This fascinating and--as far as I know--unique wrinkle combined with Wordsworth's focus on the self to produce "brag rap," perhaps the most common style used throughout the history of the genre.
Whereas Wordsworth used his writing ability to express his inner self, rappers use their lyrical skills to help you understand just how dope they are. Rappers know that if their lyrics don't hit home the first time someone hears them, the listener is unlikely to give their music a second chance. The result is rappers having to come up with quick, clever, ear-catching ways to talk about themselves.
The best brag rap lines usually involve a comparison of some kind, are sometimes extremely strange, and sometimes extremely specific.
Big Boi is both strange and specific when he says he's "cooler than a polar bear's toenails."
Joey Bada$$ keeps things unexpected when he says "you gotta drown a fish before you clown me."
Jay-Z is so self-assured that all he has to say is "Google me, baby," for you to get the idea just how big of a deal he is.
My favorite lines come when rappers assert their lyrical or poetic superiority over the competition. UKG does this as he combines the high and low-brow when he raps, "Go read a book, you illiterate son of a b****, and step up your vocab."
There have been a lot of important people in the history of hip-hop. But as unexpected as it seems, William Wordsworth laid the foundation that our favorite rappers stand on today.