For me it is Andre Rublev http://en.wikipedia.org/wiki/Andrei_Rublev_(film)
Why do I like Andre Rublev better than all other films?
I like it best because it comes closest to portraying a whole world.
Most films are slices of the world which they portray. Few films strive to portray the whole world not just a slice, or several slices of it.
Why is it existentialist?
See http://wp.me/pPsy-as