I'd choose 'Baby Birch', I can't remember another song with its intensity, emotion and feeling. Lirically it is so deep and profound, musically very rich and touching. The progression of the last part is the most beautiful thing I have ever heard IN A WHILE. Probably one of my favorite songs ever. And the final phrase "be at peace, baby. and be gone" gives me goosebumps every time I listen to it.
Then, I want to mention this songs:
- 'Bridges and Balloons' - this was the first song I really loved by Joanna Newsom, and I think you'll agree with me that its melody is one of her bests.
- 'Only Skin' - Ok, I don't have a clue about what is she saying (maybe something about the relationship between son and mother or sth like that), but it's clearly one of her most powerful songs in terms of arrangements and melody. Also, and as Baby Birch, this song progresses to a fabulous climax that suddenly vanishes like nothing has happened. The first line of the song really touches me.
- 'Good Intentions Paving Company' - This is simply brilliant, a masterpiece and the song of the year. It is also my third most listened song on Last.fm. I think this song is her catchiest ever.