None of them made an impact in society as big as Eminem did. Dre was great, but when he signed Eminem he became THE rap Godfather. A white man coming up in that industry, cleaning house and being labeled as the greatest hip hop artist to ever hold a microphone was pure fiction until he changed that. Also, RIP to the deceased - he didn't even have to die to make the impact that he did. For example, Tupac. Do you think he would have been as big as he was if he didn't die at the time that he did? I don't and neither did he. Listen to his lyrics.
And the ball cap example was just an example. It has nothing to do with being a music icon, yes, but it does provide proof of what he changed and his impact. It wasn't solely him who changed that, but he sealed the deal.