def was Run DMC, selling 3 million+ with Raising Hell and Walk This Way is the one that really broke down those walls. there were people all over the world who were into Hip Hop due to uncles or themselves serving in the military or a family member who went to NY & discovered the music & brought it back or even just from experiencing movies like Wildstyle & seeing concerts & performances back in the day. they had the Rock Steady Crew going all over the world performing for the Queen & shyt in 1980/82 or something.
BUT the thing that brought it to EVERYONE was RUn DMC, no doubt. after that i think it was Dre & Snoop/Death Row that solidified it & made it even bigger because Hammer & Vanilla Ice although they were big, they were accepted by the pop audience in a way that NWA or Public Enemy were.
Wu, Death Row & Bone Thugs were all the leaders of the 90's which was what made Hip Hop as BIG as it is today but if it wasn't for Run DMC breaking down those doors and if it wasn't for groups like Public Enemy who had a lot of rock heads listening to them, then Dre & Wu would have nothing to grow on. Beastie Boys had a lot to do with attaining the crossover rock crowd too.