Yeah, I hate making lists too. I tend to like non-conventional screenplays. I give credit where credit is due, but I'm not going to say, "Casablanca is Duke of Hollywood, A NUMBER ONE!"
You know I like my Kubrick screenplays, which tend to feature very little dialogue but pay insane attention to the visual details of the scene. That's like, by definition, what a screenplay SHOULD NOT be, but I'll be damned if I don't like reading me his Napoleon screenplay.
Also, Taxi Driver is one of my favorites, but that movie has a significant amount of improvisation in it. How do I count that when determining how well written a screenplay is?
Fritz Lang's M would also be one of my contenders. . .but it's in German, so I don't feel adequate judging its quality since I am not familiar with the language.
I also think Chinatown is a somewhat "safe" pic. I think Polanski's earlier films are much more interesting. Nobody could play a psychological angle like that man. Chinatown makes it on all sorts of best lists, and while it is a great movie, I don't think it ranks as one of Polanski's best. Jack is fuck awesome in it, though. <3 Jack. Rosemary's Baby would maybe be my pick. How many truly well-written horror movies are there? Not many! Hmm, maybe I would pick Repulsion. So hard to pick just one.
Oh shit, Network is a good pick for a well-written screenplay. Amaaaaaazing movie.