Jimi Hendrix, Kurt Cobain, and Nick Drake (can't believe noone has said Nick Drake already) were all right in their prime when they died. Their last studio albums were Electric Ladyland, In Utero and Pink Moon respectively, not only 3 unbelievably fantastic albums, but arguably each artists best release.
Cash, Mercury, Bonham and Dio, although all great artists in their own right, were definately past their peak when they died.
On the other hand, I'm not really sure I'd revive any of them. I would hate to see Hendrix or Cobain becoming dull and old or horrible self-parodies like so many of the decade spanning careers we see. Remember the Who's superbowl performance. Now imagine replacing them with a 60 year old Hendrix and then consider whether it actually turned out for the best or not.