A Robert Heinlein biography is on the way

Not long ago, I was commenting on how I would like to see a collected letters of Robert Heinlein. That might not be coming any time soon, but an authorized biography is coming soon on the man who many consider to be science fiction's greatest author.

The first volume of the biography is coming out in August and will be titled Robert A. Heinlein: In Dialogue with His Century: Volume 1 (1907-1948): Learning Curve. To get people excited about the release, the publisher, which is Tor, has asked a number of prominent science fiction authors to name their favourite Heinlein books.

The first response to be posted is from David Brin who cites Beyond This Horizon as his fave. I can't say that I've read that one, but I have certainly read many of Heinlein's books. I cannot decide between The Moon is a Harsh Mistress and Starship Troopers. Both are fantastic books.

So what is your favourite Heinlein?