I really like this thread. So, another bump:
Today, I recommend
Bringing Up Baby (1938) directed by Billy Wilder and starring Cary Grant and Katharine Hepburn. It's a screwball comedy about a socialite (Hepburn) who teases/messes with a paleontologist (Grant). The dialogue is very quick and crisp. They are both (Grant and Hepburn) hilarious.