Hands down, Minority Report. I've seen it a gazillion times and it never fails to chill with its superb interpretation of my ultimate nightmare: one in which corporations and governments conspire to control every aspect of our lives, all in the name of safety and the common good (and perhaps I'll never get to eat Cheetos again because they're all chemically and stuff.)
It also proves just how good a storyteller Steven Spielberg really is—and how easy it is to forget that. Maybe it's his public persona—all gnomic and Hollywood slick—that makes us mostly associate him with treacly suburban American fairy tales and bombastic action-adventure blockbusters rather than anything more hefty. We forget the darker stuff and its lineage: Duel, a first rate made-for-television psycho thriller that was Spielberg's first movie, followed by the gritty Sugarland Express, and a few years later the squick-inducing Jaws. All long before that stupid little alien dude marooned himself in Los Angeles county and sent the demand for Reese's Pieces through the roof.
But a list of Spielberg films shows that the heavy far outweighs the lite. Sure, good, or at least justice, eventually prevails, regardless of how dystopian his initial vision (even the unrelenting horror of War of the Worlds ends sunny side up), but the journey he takes you on before reaching that destination is one of the most thrilling in modern movie-making.
Once again, YouTube twarted my efforts to post the best clip available (http://www.youtube.com/watch?v=q2bmImPNKbM), but this clip is equally as interesting: