It's the one named after a city in Australia. Justin Vernon explains to Exclaim!:
The first thing I worked on, the riff and the beginning melodies, was the first song on the record, 'Perth,'" Vernon says. "That was back in early 2008. The reason I called it that right away, is because I was with a guy that I didn't know very well, but basically, it's a long story, but in the three days we were supposed to spend together — he's a music video maker — in those three days, his best friend [Heath Ledger] died. And his best friend was from Perth. It just sort of became the beginning of the record. And Perth has such a feeling of isolation, and also it rhymes with birth, and every song I ended up making after that just sort of drifted towards that theme, tying themselves to places and trying to explain what places are and what places aren't.
The music video in question appears to be"The Wolves (Act I & II)," and the director in question appears to be Matt Amato, Ledger's friend through the art collective the Masses. Also: "every song I ended up making after that just sort of drifted towards that theme"? So basically the whole album is sort of about Heath Ledger?