If you could bring back one great artist that has

Janis Joplin and Amy Winehouse - both had voices and material to develop and mature

I can't believe no one mentioned the chairman of the board.

Frank Sinatra and Jim Morrison.

Paco de lucia, the greatest flamenco guitarist. And, of course, El Camaron de la Isla - the greatest flamenco singer. Last ten years of their careers there was nothing to write home about though. Same with Miles Davis.
But they were all the very best, I miss them.

Chris Whitley
Jimi Hendrix
John Entwistle
Jaco Pastorious
George Harrison
Duke Ellington
Count Basie