have to disagree with the view Amy Winehouse
was at her peak when she died in 2011. Sadly alcohol and drugs had taken her toll upto then and in her final years she was more infamous, for her shambolic live performances under the influence behaviour. I agree she had a lot more to give, but for me the circa-2006 Amy was her at her peak....5 years before she died. This is the Amy i prefer to remember
Similarly Freddie Mercury. His best work was late 70s and very early 80s. After 'Hot Space' I think Queen declined critically (most Queen fans put their mid to late 80s albums to the bottom end of the 'best' albums list), and from the mid 80s their US sales dried up, again I think Freddie had a lot more to give but he peaked just over a decade before he died IMO.
Then again I do recall interviews in which he said 'Barcelona' (his 1988 solo album) was the record he always wanted to make. Even if that album wasn't commercially/critically successful as his Queen work, from his point of view he would probably say he went out not long after reaching his personal peak
Would agree with Heath Ledger and Kurt Cobain though
To be honest after A Kind Of Magic album, Queen seemed to hit their form again with The Miracle and Innuendo. I really liked the Barcelona album even though I don't listen to Pop Opera that much. I would have liked Freddie to have done more Opera as it would have been great, but as you said his time had ran out.