This is a great list so far. I have to see Sanjuro still. I am a huge Billy Wilder fan (well, I haven't seen the later stuff). I used to say Sunset Blvd. was my favorite film but I find it hard to really come up with a favorite anymore. Have you ever seen One, Two, Three? It has some of the best comedic dialogue delivered at a rapid pace. The Berlin Wall was erected while they were in the middle of filming it, which made the production have to move. It's kind of fascinating as it depicts a post-war pre-wall Berlin that I never really knew about before. It was probably mostly forgotten because it's such a specific time that people largely forgot about.
I do a Tuesday movie night at my house every week in the fall and winter. We watched The Apartment a couple years ago and everybody loved it. It's one of his best. I also really love Lost Weekend and Double Indemnity.