I'd say The Tain is my favorite Decemberists song, but uh, that's sort of a cop-out. I wasn't too wild about the new songs, but I've only listened a couple times, and most of their stuff takes many listens for me to get into it. If I had to order my, like, 5 favorite...
1) A Cautionary Song
2) The Legionnaire's Lament
3) Leslie Anne Levine
4) The Chimbley Sweep
5) Los Angeles, I'm Yours
But if you wanna cut it to brass tacks, I pretty much like all of their songs (I'm not so big on "I Was Meant for the Stage" or "California One/Youth & Beauty Brigade), and I totally want Colin Miloy to impregnate me.