So I voted for the Martian Chronicles (among others). It seems a lot of us did. And while I really don't think it's the best book in this group, it is really good, and really, really fun. It's ideas of what Mars are like are dated to say the least, but that just adds to the fun. Just another in a long line of pretty mars fantasies, from John Carter's Barsoom to Stranger in a Strange Land.
And the book's form is quite interesting. Really just a collection of short stories, each very personally focused, but they have a direction and tell a larger story that transcends individual characters and brings the scope up to societies and worlds, a future history. This is the sort of thing that Science Fiction does best.
And of course it's sobering to note that though the stories were written in the 40's, the future they depicted is mostly now in the past. The first colonies on Mars were to have been founded in 1999, and most of the book covers the period between then and 2005. The last three are set in the far future of 2026.