What's your favourite classic movie you think everyone should have seen once in their life?

Me and two friends had "classic movie nights" for a couple of years before I moved away. We would watch something which is considered a classic and it had to have been released before 2000. We watched only those which none of us three have seen before and we would watch it like once every two months or so. Movies like:

• M
• Gone with the Wind
• The Godfather
• Taxi Driver
• Murder on the Orient Express
• One Flew Over the Cuckoo's Nest
• Rear Window
• Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
• Chinatown
• Le Grande Bouffe
• L'Avventura
• Tengoku to jigoku
• etc.

It was a ton of fun and we talked about the movie before, what our expectations are and after just generally and each of us would give it a IMDB star rating.

Now sadly my friends live 9 time zones away, so we can't really do that anymore. But I was thinking to try to convince my wife to do this classic movies night with me. Right now she is reluctant because English is her 4rth language and especially older movies are using language differently too, but one day she will give in :D.

Anyway, now that you know the rules, what movies do you think I still missed and should watch?