Shirley MacLaine, at the end of the Apartment, after Jack Lemmon's effusive declaration of love:

"Shut up and deal."

Best last line ever?

theoretically it's 7 words, isn't it?
With "I'll" and "she's"
(I'm splitting hairs ;)

@Steve: I'm extending this theme through next week. That will surely be included. I agree. Perhaps the best last line ever.

@Marc: I'd like to say that I was being clever and testing all your grammarians, but no - you busted me. A lesson there, however, how much better is "I'll have what she's having" than "I will have what she is having." The value of contractions! [Of the non-labor kind].

That's right. Sounds a lot better. It is something to think about when writing dialogue. And one more reason to read your lines out loud. Good point. I'll take it to heart and let it pass this time ;)

And I have another one:
5 words, 2 characters, maximum impact:

Princess Leia: I love you
Han Solo: I know