Ryan H. at 2010-05-12 09:14:17:

Plays are particularly helpful in this regard. See, for example, THE LION IN WINTER or WHO'S AFRAID OF VIRGINIA WOOLF?, both of which were faithfully adapted into spectacular films.

The Unknown Lyricist at 2010-05-12 09:15:24:

"American Beauty" & "Philadelphia Story."

Tom Murphy at 2010-05-12 09:24:22:

How's about Double Indemnity (Billy Wilder & Raymond Chandler) - all that sizzling, Hays-Code-dodging stuff between Fred MacMurray and Barbara Stanwick?

Script available at http://sfy.ru/?script=double_indemnity_1944

In fact, you could probably add anything from Billy Wilder - especially The Apartment

Steve Anderson at 2010-05-12 09:27:58:

Glengarry Glen Ross
The Departed
Dr Strangelove

Stop me. I could go on like this forever.

Steve Anderson at 2010-05-12 09:30:15:

Ditto that about plays, of course, and definitely Mr Billy Wilder. I have to add one more: The Sweet Smell of Success. Okay, now I'll stop.

Peter Dwight at 2010-05-12 09:42:39:

Ordinary People. Excellent dialogue.

http://www.imsdb.com/scripts/Ordinary-People.html

Nate Winslow at 2010-05-12 10:47:39:

Inglourious Basterds. If Mr. Tarantino is good at one thing, it's dialogue.

http://www.sendspace.com/file/t7loun

Adam at 2010-05-12 10:55:55:

North by Northwest - great dialogue and IMO the most purely entertaining movie Hitchcock ever made, although Psycho is still my favorite.

http://www.dailyscript.com/movie_n-z.html

Emily Blake at 2010-05-12 12:12:30:

I've always loved the dialogue in a little unloved movie called THE PAPER.

Cake Man at 2010-05-12 12:46:17:

THE UNTOUCHABLES has some great dialogue. Every other line seems to blow me away.

Martin at 2010-05-12 14:32:34:

Barton Fink
Butch Cassidy and the Sundance Kid
Fargo
Marathon Man
The Princess Bride
The Hospital
A Streetcar Name Desire
The Untouchables (ditto @Cake Man)
The Verdict

James at 2010-05-12 15:15:38:

PULP FICTION
CLERKS
JUNO / JENNIFER'S BODY / UNITED STATES OF TARA

For adapted works:

FIGHT CLUB

J. at 2010-05-12 19:21:29:

How about some Sorkin?

A Few Good Men
The American President

BTW, new reader; first-time commenter. Enjoying the blog so far.

lonestarr357 at 2010-05-13 06:38:02:

*coughSunsetBoulevardcough*

http://www.dailyscript.com/scripts/sunset_bld_3_21_49.html

mrswing at 2010-05-13 13:57:10:

Miller's Crossing should definitely be on that list. Not only chock-full of quotable lines, but every character has a very distinctive voice.