Presuming they'd come back at age they departed, and could be cured of their major health (including mental health) problems .... Any of:
Janis Joplin, Jim Morrison, Jimi Hendrix, John Lennon, Marilyn Monroe, EA Blair, JFK, Anna Sewell, Alan Turing... and there's a guy who published science & other articles in the 1920s, his name escapes me but he was amazingly clever, died about age 30 in about 1929. Name-escapes-me guy is my 1st choice. Anne Frank of course, who would have no doubt had an ordinary life if she lived, but her death was the horribly unjust one, so feels like the most important one to undo.
I would like to see what any of them did with another 30+ years of living.