Assuming they resume at age and performance level at the time they passed, John Lennon no doubt.
Janis Joplin? Really?
Duke Ellington would also be a contender.
Jim Morrison could have been an interesting choice assuming he also cleaned up his act from what it was at the time of his death.
Gotta put a vote in for Buddy Holly also.
I think a lot of people really miss Johnny Cash as well including myself.