another vote for Jeff Buckley … what a talent he was!!!
Heh, there’s a time travel card game that speculates that given his survival, he would have become a Senator, led the charge to legalize pot, and, as an activist, stage a protest on the Challenger launch pad, preventing it from taking off and exploding.
So who knows? 
Lennon
Sam Cooke
Without question, it’s have to be Kurt Cobain. Nevermind and RHCP’s Blood, Sugar, Sex, Magic are what finally got me interested in music while in high school. To be able to see if Nirvana would have withstood the passage of time…