Most of the greatest screenplays ever written were never produced. Here is a list of some of the best;
1) Napoleon by Stanley Kubrick
This meticulously researched epic (Kubrick hired Oxford University professor and reknowned Napoleon expert Felix Markham to serve as his advisor, and set 20 of Markham's graduate students the task of compiling a complete biography on the 50 principal characters in Napoleon's life) is one of most famous examples of the greatest movie never made.
Kubrick was obsessed with the Frenchman, he read hundreds of books on him, and would borrow his manners and idiosynchracies – the most prominent of which was Napoleon's habit of bombarding every person he met with questions, a habit Kubrick kept for the rest of his life.
Malcolm McDowell tells an anecdote of watching Kubrick eat; At a restaurant he watched as Kubrick took a bite of dinner, a bite of dessert, then a bite of dinner – the director smiled at the bewildered McDowell and explained 'This is how Napoleon ate!'
The film was to be an epic. A bloody, war and sex driven character study encompassing the entire life and times of the French Emperor.
Unfortunately after Waterloo (1970) bombed, funding for Napoleon was cut and the project was never made. The script was lost for years, and eventually turned up, naturally, in a salt mine vault in Hutchinson, Kansas.
Apparently the salt mine had been used as a storage safe for years. The United Artists executive who found the script, Jeff Kleeman, exclaimed in a press interview, "The last scene of Citizen Kane had nothing on this place."
The script itself has been online for a while, surfacing shortly after Kubrick's death. You can read it here; http://donkeyontheedge.com/i/napoleon.pdf
Note that Kubrick came from the blueprint school of screenwriting. You won't find poetry here, but if you read his other scripts, you'll realise exactly how he turns his vision into reality on screen, and how epic this epic could have been.
2) I Am Legend by Mark Protosevich
Though this film was actually made with Will Smith in the lead role, this script is superior to the produced one in almost every conceivable way. For starters, it sticks closely to Richard Matheson's original story.
Well worth a read for fans of Matheson and sci-fi in general; http://www.horrorlair.com/scripts/legend.txt
3) Watchmen by Sam Hamm
I am a devotee of the graphic novel, and of Alan Moore. I enjoyed Zack Snyder's slick adaptation. I thought he handled the transition to the screen well. I even accepted his re-imagined ending. But something about it didn't work…
Sam Hamm is perhaps most famous for writing Tim Burton's Batman. In 1989 Joel Silver was trying to scrape together $100million for Terry Gilliam to direct Watchmen, and hired Sam Hamm to draft the screenplay.
What Hamm came up with would have blown Burton's Batman to pieces. It has infinitely more substance the Zack Snyder's version, and with Terry Gilliam's style could have been… well, we'll never know.
Joel Silver couldn't get $100million and the project was shelved, consigned to development hell for two decades.
You can read Sam Hamm's Watchmen here; http://www.scifiscripts.com/scripts/wtchmn.txt
4) Alien III by William Gibson
There were 30 scripts written for Alien III. Three-Zero. There was the famed 'wooden spaceship draft'. David Twohy, who went on to write Pitch Black and The Chronicles of Riddick, wrote an excellent treatment of the story.
His draft involved a prison spaceship, killing off Ripley and following a pack of prisoners who must stage an outer-space prison break while simultaneously survivng the alien terror.
Kind of like the best scene in Riddick for a whole movie. With Aliens.
But my favourite draft is from Neuromancer writer William Gibson, and it is my favourite for this one simple reasons; William Gibson wrote it.
This is the man who invented the internet, people. Conceptually anyway. No William Gibson = no matrix, no cyberspace.
His vision for Alien III is a dark treatise on the meaning of humanity, the role of darwinism in the evolution of the universe, and it properly tackled the Alien-Human hybrid long before Resurrection f-ed it up.
You can read it here; http://www.horrorlair.com/scripts/alien3_gibson.html