The best I’ve ever heard comes from the wonderful 70s sitcom ‘Rhoda’, starring the mesmerising Valerie Harper. Male pest drops by Rhoda’s apartment uninvited.
Rhoda: What are you doing here?
Pest: I thought I’d drop by and cheer you up.
Rhoda: (one beat pause for thought) You cannot do both.
Brilliant writing, brilliantly delivered.
Another one similar to the Peter Cook line is “I’m sorry, but I’ll have to cancel due to a subsequent engagement”.
Others:
“I see you’re suffering from delusions of adequacy.”
“You strike me as the sort of person who can light up any room just by walking out of it.”
“A date? Great! Just tell me when / just tell me where / and I’ll make sure / that I’m elsewhere”.
And to paraphrase Steve Martin… “Sure. And just in case hell does freeze over… what’s your number?”.
Don’t know if this would ever come up in normal conversation, but I once was hit on by a guy in a bar while I was playing a video game (it was a pistol shoot-the-bad-guy game and I’m a very good shot).
Bar Guy: (watches my accuracy) “Saaaaay… you’re pretty good! Where did a pretty girl like you learn to shoot like that?”
Me: “My boyfriend’s a cop.”
Bar Guy: “Oh.” (disappears before the next onscrren baddy pops up).
I’m leaving a bar with this hot chick[sup]TM[/sup] and two guys (buddies of mine, sort of) near the door comment:
Guy A: You’re not leaving with him are you?
Guy B: If you leave with him, you’re making a big mistake!
Hot Chick: “I’ll let you know if it was a mistake after we’re finished.”
Best un-rehearsed one liner I’ve ever personally witnessed.
Years ago, when I was in college, I was out with my then-GF and her roommate. Some drunk guy came up to the roommate and said, “Hey, what’s your name?”