V is For Vendetta. It's such a daring script and the dialog is also fantastic.
For those that haven't seen it, watch this:
http://www.youtube.com/watch?v=uW6HbZXI9Y0

The Princess Bride, hands down.

Produced
Lethal Weapon by Shane Black. It's a script that changed the rules and created a screenwriting rock star.

Unproduced
Shadow 19 by Jon Spaihts. In my opinion, it's the best sci-fi action script out there right now.

Mr. Brooks! Awesome movie!

So many, but also none (I don't want to tamper with the universe in a thieving kind of way).

Catch Me If You Can comes to mind though. So funny, fun, and seriously resonant at the same time. And the story of the two mice might be the best stating of theme ever.

You Can Count On Me

Easy:

Galaxy Quest.

Funny, fun, fast, clean, knowing, good-hearted. Beautifully put together. So well done it looks effortless.

How about you, Scott? What do you wish you'd written?

I'm not certain this qualifies, but the truth is that when Avatar came out, I had mixed feelings. My screen play idea resembles Avatar in some important ways. I've had to make some changes in order that it not look too much like an Avatar re-make.

In a way it's good news because Avatar's success encouraged me. I've got a good core idea!

I just wish that I'd finished mine sooner!

Notorious, the 1946 classic with Cary Grant and Ingrid Bergman.

It was brilliant, romantic, suave dialogue in a plot that was just rife with suspense.

My favourite lines:

Alicia: I'm terrified.
Devlin: Just pretend you're a janitor. Janitors are never terrified.


Alicia: You could have stopped me with one word, but no, you wouldn't. You threw me at him!
Devlin: I threw you at nobody.
Alicia: Didn't you tell me to go ahead?
Devlin: A man doesn't tell a woman what to do; she tells herself. You almost had me believing in that little hokey-pokey miracle of yours, that a woman like you could change her spots.
Alicia: Oh, you're rotten.

Recently.... Micmacs. Glorious and imaginative, as is always the case with Jeunet.

A little less recently... Suture from McGehee and Siegel.

A lot less recently... The Third Man from Greene.

Chinatown, Shakespeare in Love, or Miller's Crossing.

Most recently, Source Code, especially after watching the movie.

I wish I would have thought of the ending to Memoirs.

The depth of Inception...

My list could go on forever I think.

Back to the Future.

And Mr. and Mrs. Smith. :)

Mike Hodges' English Patient (1996)

Now both of those were adaptations, so for original screenplays:

Anne Fontaine's Girl from Monaco (2008)

Francis Veber's Dinner Game (1998)

And of course Joseph Mankiewicz's All about Eve (1950)

Oops. Before the html scrambled, that was meant to start with:

Mike Hodges' Get Carter (1971)

Anthony Minghella's English Patient (1996)

Don't wish to have written any as much as I wish I could write as well as:

Preston Sturges, Paddy Chayefsky. But those probably wouldn't sell today.

Either "Big Fish" or "Up".

Both of those movies made me feel like a hack! Obviously many, many more, but those two stand out for me.

Interesting answers, all.

@Nicola: I'll answer your question in an update on the OP.

Cool to see what movies really grabbed each of you. Frankly I could watch each one of your suggestions in a heartbeat.

Two James Cameron flicks: The Terminator and Aliens.

Back to the Future, The Truman Show, and Jerry Maguire.

There are too many to list, but the most immediate that comes to mind is The Sixth Sense.

Patton

Charade by Peter Stone.

Up. Pretty much perfect.

The Sixth Sense. Easily. Well, so long as I didn't actually have to be M. Night.

That movie was incredibly well done and I know of few other movies that provided such an incredible viewing experience in the theater. I have never sat in a movie theater with my mouth agape like I did with the Sixth Sense's big reveal.

You don't think The Last Boyscout was an awesome script?

If I had to choose, I'd say True Romance.

@jess_r: I liked The Last Boy Scout a lot. My point was simply to steer people to think about movies they loved as opposed to picking a script that made a ton of money.

Interesting to see Back to the Future mentioned as often as it is. That's also a candidate for a perfect movie in my book.

I had to change my original choices to these four: All the President's Men,Network, Dog Day Afternoon, or the Americanization of Emily.

Three very different ones off the top of my head: The Big Sleep - Junebug - Michael Clayton.

Oldboy as a produced screenplay.
The brigands of Rattleborge as un produced.

Little Miss Sunshine and The Visitor.

They are written clear and precise. Without any fuss. Well structured, great characters, lots of emotion, easy to read...

Of course I should add Chinatown, also brilliant but this is hardly news.

The Hangover

I always say "none" to this question. There are many films I love but I don't want to have written them.

I want to write new stuff.

Stuff where people say "I wish I'd written that" :-)

Shakespeare in Love, written by Marc Norman and Tom Stoppard.

Groundhog Day
Being There
The Sixth Sense
Garfield
Jacob's Ladder
The Truman Show

- just kidding about one of the Bill Murray movies.

Close Encounters of the Third Kind, that script is amazing.

Spiderman 3. (And 4).

I guess, I look at this question backwards from everyone else. I'd fear I'd only eff up what was great.

Spiderman 3 was awful.

I really did wish I had written the Venom / Carnage storylines for Spiderman though. I had a couple ideas outlined and some scenes written just messing around -- was pretty sure Raimi wasn't going to do Venom (because he hates the character). The studio shouldn't have forced the decision.

The symbiote / Spiderman storyline is an identity crisis. As is the Eddie Brock / Venom story. They lend themselves together very well.

And Eddie Brock / Venom is such a cool nemesis. He's literally a bigger, badder Spiderman -- and Eddie is a bigger, but less successful photographer than Peter Parker. He's got a lot to be bitter about.

Always being one upped by Peter Parker AND Spiderman.

There's a whole movies worth of conflict in that alone.

Also CHOKE.

Wish I had written that. Didn't see the movie. Loved the book. The trailers made the movie look like a pretty watered down version.

Dead Poet's Society.
So many good characters each with very different but effective story lines.

Citizen Kane.

I wish I had that sort of genius within me. So many gray areas, so much subtext, so much STORY.

Off the top of my head...

CAPRICORN ONE (Peter Hyams). The dialogue in that is so amazing. In particular, the scene where David Doyle rebuts Elliot Gould with this great, condescending speech may well be one of my favorite scenes in all of film.

Quentin Tarantino's draft of NATURAL BORN KILLERS. CHINATOWN is the best screenplay in my book, but I get giddy reading NBK.

I think "While You Where Sleeping" and "Nothing Hill". I don't know if they categorize into any best, great, glorious screenplays by any other else but me, but I don't care. They are movies that I love to see and return to over and over. And they are movies of the feel-good-type that I like. Interestingly enough I never tried to write a screenplay in this category until now.