Sam: Quiet. I'm thinking.
Diane: Somebody note the date and time.
What do you think this is, a signature? It's a way of life!

Diane:
How would you like to take a flying leap up a knuckle sandwich?
Yankees Fan: Hah?
Diane:
You gather my inference.

From the second episode of the first season (first show after the pilot):
(after the two are arguing about the kind of women Sam dates)
Sam
:(in a comeuppance tone) Well.. uhh, I've never met an intelligent woman I would
want
to date!
Diane
: On behalf of the intelligent women around the world, may I just say:
pheww
!
I saw this episode (and the whole first five years) when it first played 30 years ago a time when "intelligent women" were just beginning to be acknowledged as existing, here and there and I never forgot that line.

Diane (to Heather): That's Carla, she likes witty repartee. She's just not capable of it.

Diane: Oh, my Sammy is putting on his thinking cap! It's a wee, pointy little thing with ear-flaps!

This was one of my favorite Diane comebacks:
http://www.youtube.com/watch?v=N8ZpsumdhfE&feature=related#t=5m15s
http://www.youtube.com/watch?v=8KKvbFgMK0E&feature=related#t=15s

Sam and Diane are sniping at each other:
Sam: At least MY dates don't count the number of letters in sentences!
Diane: Your dates can't FORM sentences.
Honour thy parents. They were hip to the groove too once you know.

Dave Richards:I'm incredibly rich, incredibly good looking
Sam:And incredibly married
Diane:Oh I'm sorry to hear that
Dave:Really?
Diane: Yes I was hoping to reject you based solely on your personality
Love that one too!
Sam: I'll think of something.
Diane: Well I don't know what.
Sam: You don't have my brain.
Diane: Well whoever has it should return it.
Diane: If brains were money you'd have to take out a loan to buy a cup of coffee!
Carla: Bowlers don't sweat, Diane.
Diane: They do if they're wearing tweed!
Diane: Mr Kramer, you made a wonderful gesture to these people and now they're asking you to repeat it.
Sam: You do that to me every night.
Diane: I'm waiting for you to get it right.

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When Carla and Diane were competing as to who would make the better waitress, Diane suggests that they each give a speech why and asks Carla to speak first-
"Carla, the floor is yours, and not for the first time, I might add."

This message has been deleted.

during the sam-diane-janet triangle
janet " on the contrary, diane - sam speaks very highly of you , and as you know sam is a man of few words "
diane " well, he only knows a few "
and
sam " you're trying to rattle me,diane and i don't rattle "
diane " except when you shake your head "

Diane : "You'll have to forgb68ive Sam, this is the year his age caught up with his iq !"
Sam : "Thats years away !"

Sam:
Hey I thought you weren't gonna call me stupid anymore now that we're being intimate
Diane:
No I said I wasn't going to call you stupid while we're being intimate
Carla:
You sound like a lady who's getting tired of her teeth
Diane:
I'm tired of your teeth and all the vermicelli in between
Diane:
Your brain could find shade in the shadow of a snow pea
Dave Richards:
I'm incredibly rich, incredibly good looking
Sam:
And incredibly married
Diane:
Oh I'm sorry to hear that
Dave:
Really?
Diane:
Yes I was hoping to reject you based solely on your personality
Diane:
It's about a woman facing adversity to raise her six children
Carla:
SixI got eight
Diane:
My God you breed like a fly
Nick:
Hey I remember you. You're that blonde waitress
Diane:
And your Carla's incredibly loathsome ex husband
Karen Walker:(in the lift)Going down?
Nathan:Wow you cut right to the chase don't you!

Diane (to Carla): I like my men more masculine than you. Not much, but more.
Sam: Come in to my office and let me show you something I've managed to overcome.
Diane: Don't tell me you finally mastered the stapler.
Diane (to Sam): As us16d0ual, Mr. Malone has his brains caught in his zipper.
Nick Tortelli: I bet you never thought you'd see me again.
Diane: Change the word 'thought' to 'begged God' and you have a true statment.
Sam: (Talking about his tuxedo) Lately I've been logging a lot of hours in the old monkey suit.
Diane: You know, up until now, I never realized just how appropriate that nickname is.
Diane: Very good, Sam. And just when we were about to hand a 'For Rent' sign on Sam's forehead.