I've always been partial to this category, even over Original Screenplay. It's fascinating to know what works have inspired others to create, or how someone else's story or characters can plant a seed that morphs into something new and exciting. Think the Socialist novel Oil! focusing in on the father instead of the son to become There Will Be Blood; think the Jane Austen classic Emma becoming the modern comedy of manners Clueless; think the impossibility of JFK working as well it does, coming from two books and mountains of speculative reports, pamphlets, and rumors.
And so in 1976, cinema is made out of a non-fiction account of a recent crime, a singer's autobiography, an erotic memoir, fan fiction, and a non-fiction account of a past crime. The recent crime won: