What is Queen Elizabeth II's greatest achievement?

The Queen held a private audience to present the Queen's Gold Medal for Poetry to Grace Nichols at Windsor Castle on Wednesday.
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London (CNN)If you can't answer the question in this week's headline straight away, it probably says more about her than you.

She's done so much over such a long period of time, it can be hard to quantify and there simply is no one else comparable.
Queen Margrethe of Denmark is also long-serving and widely admired by her subjects but Elizabeth's reign has been longer and she has significantly more subjects.
    Perhaps we should first ask what the primary role of a monarch is?
      It isn't to serve for as long as possible; it's to serve well and leave the throne in a stronger position than when you inherited it. In that regard, Elizabeth has outperformed against all odds.
      She has reigned during a period of change that threatened the monarchy on many fronts. The media revolution put a largely unaccountable institution under unprecedented scrutiny and took away some of the mystique that made it so intriguing. That came with a loss of deference in wider society, which made the monarchy appear even more aloof and unrelatable.
      But the Queen has a unique ability to turn threats into opportunities and she embraced the media and used it to connect with her subjects, most famously by inviting cameras to film her on walkabouts and among the people. She televised the Christmas message and in doing so inserted herself into the most intimate family moment of the British year. She had an official social media account before any of her grandchildren.
        The Queen and Duke of Edinburgh drive past children in Sabina Park, Kingston, during their visit to Jamaica in 1953.
        But Elizabeth's greatest counterstrike has to be the Commonwealth. When she acceded to the throne, her empire -- once the largest on Earth -- was collapsing. Country after country voted for independence and many saw it as the beginning of the end of the entire institution of monarchy.
        Elizabeth, then still in her 20s, stopped the rot by throwing herself behind a fledgling group her father had overseen for post-colonial nations. The Queen made the Commonwealth a priority and, under her stewardship, it has grown from eight members to 54 today.
        Why does that matter? Because it changed the narrative. As quickly as she was losing authority with a collapsing power base, she was regaining it as the figurehead of an association of independent states. She kept her international footprint and modernized at the same time.
        The Queen opens the 2011 Commonwealth Heads of Government Meeting in Perth, Australia.