Who was one of the first artists to bring hip-hop into the mainstream music market?

There's no official answer, it's pretty much saying "Who did the first shot in the Boston Massacre?"

However, the term "Hip-Hop" was coined by Keith Cowboy, who was a rapper with Grandmaster Flash and the Furious Five.

The origin of Hip-Hop was in the 70's when there were block parties in the Bronx and the crowd was mainly African-American. 

Some would say DJ Kool Herc is the person who started it.