As determined by the Writers’ Guild of America in this list. The top ten:

Casablanca
The Godfather
Chinatown
Citizen Kane
All About Eve
Annie Hall
Sunset Blvd.
Network
Some Like It Hot
The Godfather II

The list at the WGA.org site is great because there are lots of fun facts about each of the projects. Here’s a quote about Casablanca from one of the screenwriters Julius Epstein:

“Warner had 75 writers under contract, and 75 of them tried to figure out an ending!”

–-Hollywood Hotline, May 1995

For the rest of the list, go here.