If I had to choose a handful of favorite Prine tunes, The Sins Of Mephisto would be my choice for one of his more serious songs--there's still humor there,, though. I also like Crooked Piece Of Time in this regard.
On the more whimsical side of things, The Bottomless Lake and, especially Fish And Whistle, which contains these immortal words:
Father forgive us
For what we must do
You forgive us
We'll forgive you