I realised that I have already written up two of the three winners of the retro-Hugos for Best Novel (here and herePebble in the Sky, C.S. Lewis's The Lion, The Witch and the Wardrobe, E.E. 'Doc' Smith's First Lensman and Jack Vance's The Dying Earth, in that order. I've read them all except, oddly, the Asimov, and would certainly have voted for the Vance (incidentally the only author still living either today or in 2001 when the award was made). But my tastes are peculiar.
Farmer in the Sky is
what would now be called a YA novel a juvenile (thanks,
A lot of this was already pretty standard sfnal fare even in 1950, but Heinlein fuses it all together into a coherent and literate package, which has a colossal amount of sensawunda, sufficient to keep the book going at full pace to the end and to keep its reputation alive among fans for decades. (He even manages the pro-scouting propaganda fairly discreetly, though of course this also helps underpin the gender and racial constraints of the narrative.)