Some of the best dialogue and character repartee:
Butch Cassidy and the Sundance Kid (Newman and Reford at their best)
Princess Bride (The poison chalice scene is breathtaking!)
All the Presidents Men (Your on the edge of your seat the entire time and Reford and Hoffman play it brilliantly)
Some Like it Hot (the sharpest of Wilder and Diamond's scripts, and that's saying something)
His Girl Friday (This shows why no one has come close to Carey Grant for comic timing)
Rear Window (The early conversations between Jimmy Stewart and Thelma Ritter are gold)
There's more, sure there is! But it's late and my brain is shutting down! Anyone else got more great dialogue movies?
For more naturalistic Dialogue I guess you'd have to go with Mike Liegh, but there again, that's something that comes out of rehearsal and allowing the actors to find the characters and letting the character speak in any given situation... that why it's important to make sure you have well developed and well written characters, if you can do that, they'll do the talking for you.