Some absolutely terrific suggestions here (which I'd also recommend studying alongside Casablanca, Chinatown, etc). As a exercise in how screenplays change from shooting script to finished product, it is also worth checking out The Shawshank Redemption shooting script, which has tons of notes by Frank Darabont on the process of page to screen.

Also, just because I love gothic horror, Allan Scott and Chris Bryant's screenplay for Nic Roeg's Don't Look Now (adapted from Daphne Du Maurier's original short story) is a masterpiece.