Elvis Presley the King of Rock and Roll for Rock and Roll.
He's a Icon that continues to get mentioned such as Jesse being a fan in the Sitcom Full House, used a refrence in one of the Look Who's Talking movies, and vast other movies or tv shows.
Will Smith For Rap
He has been able to have a successful singing career, and has done it without putting cuss words in his songs.
I'm hoping this is a joke.
For rap I think of a few on the top of my head:
Sugarhill Gang- Helped bring rap into mainstream America
Grandmaster Flash- Obviously
Public Enemy- They brought the rebellion rap into the mainstream
Run DMC- They did other stuff obviously, but their mash up of Walk This Way with Aerosmith was maybe the first "rap" song everybody was talking about.
NWA- They really brought the hardcore gangsta rap into the mainstream
Eric B. and Rakim- Really helped people step up their games and instead of just using a catchy drum beat and rhyming, they actually used metaphors, etc.
Tupac- Maybe the GOAT, showed West Coast rap was just as good as East Coast, but was also a crossover star. Because the first household name in the rap game.
OutKast- I'm biased, but they made Southern rap
Eminem- This can be debated, but I could argue that he brought rap to where it is today in terms of popularity and bringing rap out from just the streets, getting played on the radio, and into suburban homes.
Kanye West/Dr. Dre- Not by their lyrics, but with their production. They've helped create the modern rap sound of today.
I can agree with all of these, and I don't think Eminem can be debated. If it weren't for him in the late 90's in the wake of the deaths of pac and biggy, I don't think rap would be near what it is today, especially when you consider all the artists who claim to have been influenced by him in the genre (even if they are terrible). This isn't even counting him as the greatest white rapper ever, which is kind of a big deal no matter how we shake it.
and I can argue Kanye has a few songs and even an album or two that makes him vastly influential whenever you realize he changes what he does on almost every album.
to add to it though
Ludacris- outside of Outkast he made southern rap. He and Andre 3000 just destroyed things, and now he's a fairly decent movie star as well (along with Ice T, Redman, Method, Ice Cube, etc).
The Fugies- Yes they only had two albums, but I can't really think of a better group in the 90s (outside of NWA, though you could argue Publik Enemy possibly) for hip hop. And what they've done after, in Wyclef and Lauren's solo careers, is continue what they do very well.
and people will probably disagree with this one, but Kidrock has to be here somewhere. He was the first person to really blend Rock and Rap and Country together. Sure Limp Bizkit kind of did rock, but Rock had more good albums and great songs than fred durst ever dreamed. And if not for him, we don't see Linkin park, Ft. Minor, Saliva, or several other bands and groups with those styles.
Speaking of rap and white... The Beastie Boys. and now I'll shut up before I write a book.