I love them all but here's my attempt at a ranking.
  1. 1.
    Because it convinced us things don't go around the earth.
  2. 2.
    Because it's phases taught us more about the structure of the universe.
  3. 3.
    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. 4.
    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. 5.
    Because it is an elusive and beautiful naked eye planet.
  6. 6.
    Because it is the only planet populated entirely by robots and could conceivably be our second home.
  7. 7.
    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. 8.
    Because I saw it from our rooftop through the telescope, pale and blue, with Sophia and it was our greatest finding achievement.
  9. 9.
    Because it was the last planet visited by Voyager 2 and the first I remember seeing covered live on the news.