I love them all but here's my attempt at a ranking.
  1. Jupiter
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    Because it convinced us things don't go around the earth.
  2. Venus
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    Because it's phases taught us more about the structure of the universe.
  3. Saturn
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    Because it marked the extent man's knowledge of the solar system for a hundred thousand years. And it is as beautiful in the sky as it is up close.
  4. Pluto
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    Because I've been waiting anxiously for pictures of Pluto since I was 12 years old and we're getting them THIS SUMMER. This is the current state of the art but they're about to get way way better.
  5. Mercury
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    Because it is an elusive and beautiful naked eye planet.
  6. Mars
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    Because it is the only planet populated entirely by robots and could conceivably be our second home.
  7. Earth
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    Because it is "the aggregate of our joy and suffering" and I think unbiasedly the most beautiful and dynamic object in the solar system.
  8. Uranus
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    Because I saw it from our rooftop through the telescope, pale and blue, with Sophia and it was our greatest finding achievement.
  9. Neptune
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    Because it was the last planet visited by Voyager 2 and the first I remember seeing covered live on the news.