It’s important for writers to read quality material. You will learn so much from this alone. In terms of lessons in writing, I find that reading a script garners much more invaluable information than from merely watching the film. You’re much more aware of the choice of words and how characters can come alive just from the text.
Here are some the screenplays in my personal collection. Many of these I’ve read several times over and they’ve helped me immensely on my own screenplays.