What are the movies with the best written dialogues?

I love movies with impeccably written dialogue, and I'm always on the lookout for more. So what are the movies with the best written dialogues for you? No limitations on your selection - it can be drama, it can be comedies... anything where dialogues are the star of the movie.

Some of the ones I personally consider at the top level:

Drama: Cat on a Hot Tin Roof, Who's Afraid of Virginia Woolf?