Of all movies that you gotta watch more than once to really understand, what is your favorite and why?

Waking Life.

If you wrap your head around it after the 2nd viewing, you're doing better than most people.

It took me a few watchings of Akira to really have it all sink in.

Koyaanisqatsi

Visual poetry. Literally. It's conveying a message, it's just you have to come up with the dialogue. Beautiful work. The sequels are fantastic as well.