I'd think Pohjola's Daughter is a very good one too... but I'm wondering if the Seventh symphony doesn't make a very good distillation as well. It has the perfect, organic development, based on motifs one would never think of as particularly friendly for development; those magnificent string chorales, as exhibited so marvelously in the Sixth symphony (my other choice for this distillation thread, by the way); the darkness with the bits of sun shining through and the predominant light with the shadows covering some of it up.
So really, I'd think from the comparatively little I've heard of Sibelius' output (that being pretty much only the symphonies and tone poems) that the Seventh symphony and possibly Pohjola's Daughter and the Sixth symphony have this sort of "distillation of Sibelius' sound world."
By the way, this thread sounds eerily familiar to the "Definitive Sibelius Piece" thread also on this board... just thought to say...