It is a tie for me:
An old black and white French movie shot mostly in the dark, almost no conversation, with occasional cigarettes being lit. I don't remember the name, but I remember sitting in the theater thinking, "Something must be about to happen...". Nothing ever happened.
A documentary called "Green" that was about things growing somewhere in South America. Lots of long shots of green leaves.
So bad they were good:
Morons From Outer Space
It Came From Outer Space 3D