Jeff Buckley, no question. He released arguably the best album of the 90s (Grace) then went and fucking drowned. Only 30 years old. He coulda been one of the best to ever do it, but we're left wondering.
Here I am, upvoting comments of people who wrote answers first: Bach, Mozart and Buckley. I tend to believe second or third albums are the best ones from an artist, can you imagine if this was true for Buckley?
Love to basically every mentioned artists, but I think most of them did give the best they had while alive.