Whether its an opening line or the opening scene in the movie. One movie per post. I'll start with with the opening scene in Goodfellas - Ray Liotta is driving the car, with Robert De Niro and Joe Pesci in it. As they drive, you hear the trunk thump ceaselessly. He stops the car, and they all come out. Ray pops the trunk open, and Pesci pulls out his dagger and stabs the guy in the trunk again and again and again. Then De Niro blasts the guy with his gun. Liotta slams the trunk close. And Liotta's narration goes - for as far back as I can remember, I've always wanted to be gangster. Only Scorcese could come up with an opener like that.
The opening scene to the 2004 remake of Dawn of the Dead. Can you imagine waking up to your neighbor's zombified daughter who then turns nurs Ana's husband into a zombie and run outside to the mass zombie takeover of the city. It was the first time zombies had strength and speed until 28 Days later came along.
Apocalypse Now -- the whole helicopter scene that eventually bleeds over to whats-his-name's room with the ceiling fan blades spinning and the psychotic look in his eyes. And the line "Saigon... Sh*t, I'm still in Saigon..."