Before Eminem became mainstream in 1999, the rap genre consisted almost entirely of black artists. There was very little diversity and, as a result, some racist elements often plagued the genre. While Tupac and The Notorious B.I.G. are incredibly gifted artists, they didn't have to fight their way into the culture. Eminem not only had to prove his worth as an artist, but he had to prove his worth as a white performer in a genre composed of predominantly black artists. When Eminem eventually broke these cultural barriers, he did so with immense success. His first album was certified triple platinum, his sophomore album went platinum within a week, and he is now the best selling hip hop artist of all time with 45 million albums sold. For these reasons, he is the best rapper of all time.
While Eminem was the first mainstream white rapper, he is not solely responsible for diversifying the genre. Dr. Dre, a black rapper, played a large part in making Eminem what he is today by giving him a platform to display his artistic talents. Furthermore, rap is still a new genre and, like most new things, it is going to take time to diversify the medium. Any of the white rappers that came after Eminem could have broken these cultural barriers.