I second the votes for Bob Dylan and Johnny Cash
But if I'm adding five new ones:
Willie Nelson
Fleetwood Mac
Jay-Z
Frank Sinatra
Pink Floyd
Fleetwood Mac was already proposed, so replace with Elvis Presley. The King deserves to be represented damn it!
Alternate/Honorable Mention: The Velvet Underground/Lou Reed (they would have to be combined)
Yeah, if I could have squished in another 5 it would have been
VU/Lou Reed
James Brown
Ray Charles
Uncle Tupelo/Son Volt/Wilco
Elvis Costello
And if this is still open a week from now, I'll post another 5.