I could dispute alot of "legends" from the 80's. Being the first to do it doesnt make you legendary in my opinion. Drop your 20 and let me embarrass you
1. Sugarhill Gang - Released first successful rap record
2. Grandmaster Flash & The Furious 5 - First rap group, before Sugarhill Gang. In The Rock & Roll Hall of Fame and The Message is in The Library of Congress' vault.
3. Cold Crush Brothers - Didn't have much commercial success but if you are into old school flyers and tapes, The Cold Crush Brothers have far more flyers and tapes than anybody else from their era including Grandmaster Flash and Kurtis Blow. Their flyers and tapes are artifacts that prove they were doing the most shows in The Bronx & Harlem out of all the groups from that era.
4. Treacherous Three - Not the absolute first rap group, but they released a handful of songs in the early 80s. Besides Grandmaster Flash & The Furious 5, they probably have the second best catalog from that era.
5. Kurtis Blow - The Breaks was supposedly the first Rap single to go gold (although I suspect Rapper's Delight went gold but Sugarhill Records didn't report it)
6. Run DMC - Came out in 1983 and competley shut down everything. Just about every rapper/group fell off after Run DMC came out. They were the first group to recognize that there was a market for an album that's all Hip Hop. No ballads or R&B songs. They used drum machines and synthesizers when everybody else was using musicians. Also, they had scratching in their songs which wasn't common at the time. It's been said that Run DMC's debut album was the first Rap album to go platinum.
7. Beastie Boys - First White Rap group. Sold 4X platinum in 1986/87 (album came out in December 86)
8. NWA - Credited for Gangsta Rap. Dr Dre produced songs that sounded sonically different than East Coast Hip Hop (D.O.C.'s album).
9. 2 Live Crew - Created Miami Bass. First rap group that had deep 808 bass in every song. Also pushed the envelope lyrically with "dirty lyrics". Had a legal case that went all the way to the Florida Supreme Court. They were accused of being "obscene" and people wanted their music to be banned from stores, but they won.
10. Too Short - First rapper from The Bay to go nationwide. Also made "dirty" songs. Rapped about Street life.
11. Public Enemy - One of the first socially conscious groups. Also they sampled records in a way that nobody else was doing at the time (e.g. Bring The Noise, Rebel Without A Pause)
12. Eric B & Rakim - It's generally accepted that Rakim's flow changed the game and that he raised the bar for MCing.
13. De La Soul - Their three Feet High And Rising album was the first "abstract" album. They didn't wear gold chains or drive around in fancy cars in their videos. They were more down to Earth. They wore beads or Africa Medallions.
14. KRS One/Boogie Down Productions - The Teacha. Their first album was more "gangsta" but when DJ Scott LaRock died, KRS became more "conscious". KRS organized Self Destruction and the Stop The Violence Movement.
15. Salt N Pepa - Female version of Run DMC. Had a few mega hits, like Push It. They continued on until the mid 90s.
16. LL Cool J - First solo rapper to really blow up and have a successful career. While Kurtis Blow and Spoonie G were before LL, LL was on Def Jam, who was owned by Columbia (a major label) so he had bigger budget albums and videos. Also secured a lane with the females that lasted into the 00s.
17. Dougie Fresh - First Human Beatbox. Invented Beatboxing. The single The Show/Ladi Dadi was HUGE!
18. Afrika Bambaattaa & The Soul Sonic Force. Bambaataa was a community organizer and created the Zulu Nation. They have/had chapters all over the world. Also, Bam released handful of classic records, most notably Planet Rock.
19. Jazzy Jeff & The Fresh Prince - Fresh Prince was a good story teller and could hold his own on the mic. If you were to make a mixtape from that era you'd have to put at least one Jazzy Jeff & Fresh Prince song. But Jazzy Jeff was something else altogether. Jazzy Jeff wasn't just a DJ, he invented turntablism. Jazzy Jeff is the GOAT DJ. Listen to the Live @ Union Square routine!
20. Ice T - Rapped about gang life before NWA. We had no idea that Bloods and Crips had gangs in the West Coast and that they wore blue and red until the movie Colors came out, Ice T made the theme song for the movie.
That's 20 MCs/groups. I left out Slick Rick, Big Daddy Kane, Kool G Rap, Stetsasonic, Whodini, Fat Boys and a few other groups that aren't really legends or pioneers, they're more veterans than anything else.
edit. Also Schooly D. He supposedly made the first gangsta rap song, but he didn't really do anything with it. He had a few dope songs but no classic album. KRS and Ice T made a bigger impact even if Schooly was technically the first Gagsta rapper.