Gotta go with Die Hard on this one.
I think Die Hard fits a category of "Mainstream hugely popular movies that everyone knows and quotes...that doesn't often get it's due as a film" (see also: Say Anything). The first half hour especially is basically a Screenwriting 101 Textbook. Tightly written, characters fleshed out, every single line of dialogue reveals both character and furthers the plot, cinematography is whip-smart and a perfect combination of style without being too flashy, that effectively sets the right mood (tension, action, levity, humor), throughout the movie.
And the introduction of Carl Winslow being a cut to a closeup of his hand grabbing donuts from a 7-11? Classic.