The "why" is hard, but the "which" is easy, although this is a 'controversial' work:Jared wrote:....please tell me which of Mozart's works below K300 would be your favourite, and if you can, tell us why.....
Plus it's only about 10 minutes prior to K. 300 anyway.
K. 297b, Sinfonia Concertante for Winds
Controversial, because there isn't clear agreement on whether it's really by Mozart or how much of the common version is by him. The current consensus (especially based on work by Robert Levin) seems to be that it is Mozart but that we don't have his own actual version of it and that the common version is a bit of a hack job, although perhaps partly on purpose to make it playable by not-so-great players.
Maybe indirectly that's part of the "why." When I first heard it, I think I sensed there was something ODD about it.
If I had to say what's my favorite Mozart from way before K. 300, I wouldn't know. So, perhaps, good point. But it still seems odd to say he's not even among the top 10 prodigy composers.