I've been thinking about this a lot lately. At this stage of my life, I can't possibly choose one.
Strong contenders? 2001: A Space Odyssey because it was highly engaging and introduced me to Arthur C. Clarke's strange world of evolutionary midwives and infinite possibilities;
Life of Brian because it was smart, silly, and critical of human belief. Not to mention having one of the best endings in the history of film; and
Synecdoche, New York for its profound artistic ambition which is all too often written off as self indulgent pretension. It is convoluted, chaotic, and clumsy by design, which makes for a difficult but ultimately rewarding viewing experience. Dig deep and you'll find a distinct logic at its core, totally unlike anything I've seen before.
Among my least favorite films are SHINING THROUGH and I AM SAM.My least favorite movie experiences usually involve somebody talking. What's that Darren Aronofsky film with the tree? I forget, but I do remember two old...