I guess it is a toss up between Senna, Moon, and Good Night, and Good Luck for me.
In no particular order: Dredd, The Thing, Alien, Aliens, Escape from New York, Robocop and Star Trek II: The Wrath of Khan. I'm on the fence if I want to add The Lives of Others or not.
All of them have themes, music and atmosphere that resonates well with me. I also love to hunt the new little things that I must have missed previously and those movies have them in spades. Or maybe I just suck at spotting things. :P
I'm not much of a moviegoer, though. I seem to lack the attention span for many of them. More often than not, I'll end up watching something because I need a motivator or breather.