Answer by Ken Miyamoto:
Yikes. The eternal question that sits just behind "What's the best movie ever made?"
The problem with these questions (And I get and love that it's more about starting a discussion and not really about finding an answer) is that A) Film is subjective, so one's best screenplay is another's overrated boring tripe, and B) There are different genres to consider.
You also have to look at the context.
While Casablanca is highly touted, most people don't know that it actually had upwards of twelve to sixteen writers. The studio system ruled the day back then and writers came and went, leaving their own contributions. Four teams of writers were hired to rewrite that film while they were shooting. So I'd moreso chalk it up to great filmmaking with a lucky collage of writing in the end.
- You've got different genres.
- You've got different core audiences.
- You've got Memento, which had a structure like virtually no other and managed to tell an amazing mystery.
- You've got Pulp Fiction, unlike anything else at the time, when you consider the new age of dialogue, the unique structure, the excellent character arcs, etc.
- You've got The Godfather II, which is one of the most flawless screenplays I've ever seen. But at the same time, it bores the hell out of many people as well.
- You've got Adaptation, which is a lesson in screenwriting itself and the very neurosis that screenwriters struggle with. The brilliant third act, in which Kaufman himself goes against all he believes a script should NOT be, and we see that the script, and Kaufman himself, needed those elements that he detested. Brilliant.
- You've got Fargo with its tension filled, yet humorist gaze into the lives of both corrupted souls, and pure goodness.
- You've got Unforgiven, taking the "go to" western formula to literally the next level, as we see a former gunslinger in the twilight of his life, taking on one last crusade against evil… and maybe becoming evil himself once again to do so.
- You've got Groundhog Day, a hilarious and unique take at comedy. High concept scenario with near perfect execution.
- You've got The 40 Year Old Virgin, which changed the way comedies today were made. Gone were the days of gross out humor. It paved the way to a new type of comedy that utilized real thoughts, real feelings, and turned them on their head with bromance (and romance) charm.
- You've got The Shawshank Redemption, perhaps the best novel adaptation ever made.
- You've got The Graduate, Butch Cassidy and the Sundance Kid, Annie Hall, Chinatown, It's a Wonderful Life, When Harry Met Sally, The Usual Suspects, and so so so many more…
But in the end, the question should not be about "What is the best screenplay ever written." Instead, it can only be "What's the best screenplay for the mood I'm in." Do I need to laugh? Do I need to cry? Do I need to cheer? Do I need to be inspired? Do I need to see shit blow up? What's the best screenplay, and eventually the best movie, for any of those?
Preaching aside, if I had to really pick one… Die Hard. It'll shock most, but this is a screenplay that served multiple needs. It had mad skills with action set pieces, it had great comedy to make us laugh, it had drama, it had touching moments, it had romance between John and Holly, it had thrills, it had suspense, it had wonderful characters, and it was even a damn good Christmas movie to boot!
No other script, in my humble opinion, above or in other answers to come, accomplishes so much within one screenplay. Lethal Weapon may be a close second in that respect.
The true answer is that there is no answer, but it starts a great discussion for sure!