The Greatest Scene Ever?
I found this on the Onion AV site talking about the best cameos of all time - certainly Alec Baldwin steals the scene here in what is undoubtedly the penultimate, signature scene from the film Glengarry Glen Ross. Indeed, it might be one of the best scenes in any film, ever.
While we're talking about great cameos, suprised that Christopher Walken in True Romance didn't get mentioned...another scene that steals the film.
And both are, of course, NSFW from an audio perspective.