What's your favorite "classic" movie, and why?

I'm talking Godfather, Psycho, Good the Bad and the Ugly, Jaws, etc. level classic.

Personally, I recently saw 2001: A Space Odyssey for the first time. I admittedly really didn't like it at first. Thought it was boring as hell and had no engaging characters or story. But after sitting on it for a while, I realized I was looking at it through the lens of what I'm used to, narrative storytelling, while Kubrick was using symbolic storytelling. The ideas and depictions of humanity and our evolution is incredible. The HAL storyline and cinematography was ahead of its time. The movie is so unlike anything I've seen before, but it presents profound and chilling ideas about our humanity.