I really think Nirvana was just an MTV band. Without MTV, there would be no Nirvana. I hate that channel.
I guess I should add that I do like Dave Grohl though. I'm not really into the Foo Fighters, but he seems like a pretty cool dude.
Elvis/Beatles/Black Sabbath....Couldn't have said it better. To add a couple more, I'd go with:
4. Eric Clapton - Blind Faith, Cream, Yardbirds, solo work. You don't have to like him, but his guitar work has been an influence to guitarists everywhere.
5. The Blues. Every band mentioned above has been influenced by The blues. I'm not really in the know to pick one blues master, but without the blues, things would have been a lot different.