Let me break down a pioneer......a pioneer generally is not the first to develop a concept, yet they are the first to generally break ground on a concept and then have that change catch on into something completely new, with that said most of the names mentioned were close to being Pioneers....here's my list of who I consider pioneers
James Brown (think about it...this guy must be the key to all things funky...I mean you had people back in his day imitating his style and to this very day, people still sample James Brown)
Cold Krush Brothers
Sugar Hill Gang
Grandmaster Flash
Marley Marl
Malcolm Mclaren(Buffalo Girls Go around the outside) this record by far was not the first in it's concept but it went over big (1982) well before Run DMC, bringing the sound of scratching an turntablism to the forefront because of the airplay the record got outside of the NY area....
Run DMC
WU Tang Klan (The most successful Hip Hop crew Period, probably the most know worldwide name maybe besides Run DMC associated with hip hop, but they were able to bring a totally new sound at that time to hip hop....runner up is Tribe Called Quest)
Pete Rock(understudy to Marley but still able to etch out legendary status as far as Hip Hop is concerned)
Stevie Wonder, Prince, Jimmy Jam and Terry Lewis were Pioneers too, you may not think Hip Hop but these guys exploited MIDI music production and sampling, and probably their concepts led to the development of a lot of the Gear and Software we utilize....I remember Jam and Lewis discussing how they used the MPC-60 to do all of the drum Sequencing on the first Janet Jackson Album....but Stevie Wonder produced almost entire albums based upon the single producer concept.....these facts can't be ignored either.
Oh how can we forget Roger Linn????????????
here is a nice interview with the guy giving birth to the MPC
http://www.sonicstate.com/articles/article.cfm?id=71&page=1
anyhow I know that some names may have been skipped or missed, but that's what i can think of right now.
Oh you can quote me on this because they are all my words and thoughts....LOL