Howard Hawks, maybe the best director Hollywood has ever seen, took a screen play by William Faulkner based on a Raymond Chandler book, and made maybe the best movie of all time. Two stars with chemistry in their prime (Bogart and Bacall), a great plot, crackling dialog, and a great pace make this a cinematic treat 67 years later.
Roger Ebert talks about what makes it such a classic:
As for the 1946 version that we have been watching all of these years, it is one of the great film noirs, a black-and-white symphony that exactly reproduces Chandler's ability, on the page, to find a tone of voice that keeps its distance, and yet is wry and humorous and cares. Working from Chandler's original words and adding spins of their own, the writers (William Faulkner, Jules Furthman and Leigh Brackett) wrote one of the most quotable of screenplays: It's unusual to find yourself laughing in a movie not because something is funny but because it's so wickedly clever. (Marlowe on the "nymphy" kid sister: "She tried to sit in my lap while I was standing up.") Unlike modern crime movies which are loaded with action, "The Big Sleep" is heavy with dialogue–the characters talk and talk, just like in the Chandler novels; it's as if there's a competition to see who has the most verbal style.
Joel Vogel on IMDB outlines the complex plot:
Summoned by the dying General Sternwood, Philip Marlowe is asked to deal with several problems that are troubling his family. Marlowe finds that each problem centers about the disappearance of Sternwood's favoured employee who has left with a mobster's wife. Each of the problems becomes a cover for something else as Marlowe probes.
Here's an example of some of the dialog: