I can't do just one.

James Coburn in The Great Escape, hanging out while Charles Bronson pretends to shower to hide a tunnel entrance, responding to a German guard's accusatory questions, delivered in a great deadpan:

"I'm watching him. I'm a lifeguard."

Kirk Douglas in Paths of Glory talking to a General who both allowed three of Douglas' men to be executed and offered Douglas his newly fired commanding officer's job:

Colonel Dax: Sir, would you like me to suggest what you can do with that promotion?

General Broulard: Colonel Dax! You will apologize at once or I shall have you placed under arrest!

Colonel Dax: I apologize for not being entirely honest with you. I apologize for not revealing my true feelings. I apologize, sir, for not telling you sooner that you're a degenerate, sadistic old man, and you can go to hell before I apologize to you now or ever again!

Pick your poison from Dr. Strangelove:

"Gentlemen, you can't fight in here! This is the war room!"

"Mr. President, we must not allow a mineshaft gap!"

https://youtu.be/RZ9B7owHxMQ

And I guess this is as much a historical quote as a movie quote, but it's too good to leave out:

https://youtu.be/NDAWaYu-YCo