Without a shadow of a doubt the best soundtrack I have ever heard is to a truly dire film, "Natural Born Killers". I've never heard anything like it, but it is an absolute masterpiece. Produced by Trent Reznor of Nine Inch Nails it mixes together pretty much every genre of music going, from Patsy Cline through Leonard Cohen, to Nusrat Fateh Ali Khan, Nine Inch Nails and Diamanda Galas, all with dialogue and sounRAB from the film binding it together. Simply stunning.
