A few I'd throw in:
Lou Reed: The Velvet Underground was hugely influential. His influence has waned since then, and some of his solo stuff is crap, but his 'voice' is oft imitated and played a major role in the direction of popular music.
Joni Mitchell: Groundbreaking singer/songwriter. Gotta throw a woman in the conversation somewhere...
Danzig: How can you not include the Satanic Elvis in the conversation?
