AFI #001 Clark Gable as Rhett Butler from Gone with the Wind (1939)

Rhett: It seems we've been at cross-purposes, doesn't it? But it's no use now. As long as there was Bonnie, there was a chance that we might be happy. I liked to think that Bonnie was you, a little girl again, before the war, and poverty had done things to you. She was so like you, and I could pet her and spoil her, as I wanted to spoil you. But when she went, she took everything.
Scarlett: Oh, Rhett, Rhett please don't say that. I'm so sorry, I'm so sorry for everything.
Rhett: My darling, you're such a child. You think that by saying "I'm sorry," all the past can be corrected. Here, take my handkerchief. Never, at any crisis of your life, have I known you to have a handkerchief.
Scarlett: Rhett! Rhett, where are you going?
Rhett: I'm going to Charleston, back where I belong.
Scarlett: Please, please take me with you!
Rhett: No, I'm through with everything here. I want peace. I want to see if somewhere there isn't something left in life of charm and grace. Do you know what I'm talking about?
Scarlett: No! I only know that I love you.
Rhett: That's your misfortune. [turns to walk down the stairs]
Scarlett: Oh, Rhett! [watches Rhett walk to the door] Rhett! [runs down the stairs after him] Rhett, Rhett! Rhett, Rhett... Rhett, if you go, where shall I go? What shall I do?
Rhett: Frankly, my dear, I don't give a damn.

- Clark Gable as Rhett Butler from Gone with the Wind (1939)

The bolded line is ranked #1 in the American Film Institute's list of the top 100 movie quotations in American cinema.

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http://en.wikiquote.org/wiki/Gone_with_the_Wind
American Film Institute's list of the top 100 movie quotations