A Clockwork Orange, hands-down. Never before have I been so impressed by a movie. Usually, I have a few puritanically conservative ideas about nudity, but in this movie it seemed perfectly necessary and acceptable (for the presentation, that is).
What probably sold me the most on this movie was the character Alex. Out of all the movies I've seen (which is to say, not that many) I've finally found one with a protagonist to whom I can relate. Yes, I'm a horrible bastard, too. While I don't go around commiting rape, murder, assault and battery, or any combination thereof, I still relate to Alex because, like me, he loves that old ultra-violence (don't we all?), but is also intelligent and cultured. Another object worthy of note is that (in the movie, at least) Alex DOES NOT REPENT for his actions. I'm sick and tired of movie protagonists who become convinced that what they've been doing all along is wrong and they change teams (this curse afflicts all members of my Modern Movie Super Triumvirate: Avatar, District 9, and Daybreakers). I like my protagonists to stick to their guns and not to apologize for being who they are, regardless of how the public responds.
Then of course the presentation is excellent, too, with all the upbeat synthetic music, the use of color, the way the scenes are set up (I'm sure a film critic could state that more eloquently, but I sure can't; if you've seen the movie you'd know what I mean), the strange costumes and characters, and of course, the wierd future-slang Nadsat.
Alone, these elements can make good movies, but put them all together and the end result is a glory to behold. I think A Clockwork Orange might be my favorite movie ever, in fact.