AFI #078 Keir Dullea as Dave Bowman from 2001: A Space Odyssey (1968)

Dave: Open the pod bay doors, please, HAL. Open the pod bay doors, please, HAL. Hello, HAL. Do you read me? Hello, HAL. Do you read me? Do you read me, HAL?
HAL: Affirmative, Dave. I read you.
Dave: Open the pod bay doors, HAL.
HAL: I'm sorry, Dave. I'm afraid I can't do that.
Dave: What's the problem?
HAL: I think you know what the problem is just as well as I do.
Dave: What are you talking about, HAL?
HAL: This mission is too important for me to allow you to jeopardize it.
Dave: I don't know what you're talking about, HAL.
HAL: I know that you and Frank were planning to disconnect me. And I'm afraid that's something I cannot allow to happen.
Dave: Where the hell did you get that idea, HAL?
HAL: Dave, although you took very thorough precautions in the pod against my hearing you, I could see your lips move.
Dave: All right, HAL. I'll go in through the emergency airlock.
HAL: Without your space helmet, Dave, you're going to find that rather difficult.
Dave: HAL, I won't argue with you any more! Open the doors!
HAL: [almost sadly] Dave, this conversation can serve no purpose any more. Goodbye.

- Keir Dullea as Dave Bowman from 2001: A Space Odyssey (1968)

The bolded line is ranked #78 in the American Film Institute's list of the top 100 movie quotations.