Mostly every song on the soundtrack is pretty good to me. The obvious choices being "Red Right Hand" by Nick Cave & The Bad Seeds, "Youth of America" by Birdbrain, and "Whisper to a Scream (Birds Fly)" by Soho. I actually really enjoy Gus Black's cover of Don't Fear the Reaper as well. As far as the original score by Marco Beltrami, I absolutely love "We All Go A Little Mad", especially around the 4 minute mark, during Billy & Stu's reveal scene in the kitchen. Such great music all around.
"This isn't a comedy, it's a horror film. People live, people die, and you'd better start running."