The very long pause before Carol Burnett says: I saw it in a window and I just couldn’t resist.
From stage (and video):
Didi Snavely of DiDi’s Used Weapons in Tuna, Texas has one of my favorites. When she gets frustrated she starts her trademark curse
"God …
takes a drag on ciggie
…
…
exhale
…
… damn"
Maybe not the best, but one of my favorites, from It’s a Mad Mad Mad Mad World: Jimmy Durante kicks the bucket, literally, and it tumbles its way down, down, down the canyon as the other guys look on, until finally you can’t hear it anymore. Jonathan Winters slowly stands up . . .
[pause]
“That guy’s dead.”
(One of those “you had to be there” moments, funny mostly because of his timing and delivery.)
I’m surprised to see The Simpsons mentioned but not the Cape Fear-sppof episode. When the family is sitting with the FBI in the room while they agree to go under witness protection, and the agent is trying to explain, again and again and again to Homer that his new name is Mr Thompson.
Agent: Now, when I say “Hello Mr Thompson”, and stamp on your foot, you say hello.
Agent says “Hello Mr Thompson” and stamps on his foot loudly and repeatedly
pause
Homer (leaning back to whisper loudly to other agent): I think he’s talking to you.
Best pause ever, on The Simpsons at least.
One more…from Airplane…
paraphrasing from memory:
Ted: “We’ll have to come in pretty low”
Elaine: “Is that dangerous?”
Ted: “Of course it’s dangerous, coming in low is part of every textbook approach. It’s just something you’ve got to do when you land.”
Elaine: …pause…“Huh?”
On an old, old, old SNL sketch, Gilda Radner played a little girl who was put to bed, but repeatedly calls for her parents to come into the room for an escalating series of “bumps in the night.” First it’s a pile of clothes on a chair. Then it’s a repairman who enters her room accidentally. After that it’s the tribe of gypsies(!) her dad allowed to live under her bed. Finally, her exasperated parents tell her that if she calls them for one more false alarm, they’ll cancel her sleepover party she intends to have. Then, Gilda discovers a blood-soaked, crazed axe murderer hiding in her closet.
The pause that Gilda makes as she weighs the pros & cons of being attacked in her sleep against having her sleepover party cancelled(!) is truly classic.
Jack Lemmon and Walter Matthau are not only funny on screen, but off.
During the making of a movie, Out to Sea, ( I think thats it), Matthau slipped on something and slid down something, like some stairs or a safety slide that was there. I can’t remember.
Lemmon was the first one too his friend, laying on the ground in a heap asking his friend after making sure he wasn’t dead and getting him into a better position… " Are you comfortable?"
Matthau: " Eh, it’s a living."
The other story about Matthau involved Matthau and his wife on vacation in Germany. They had gone to visit a Concentration Camp and somewhere after it they had a rather nasty fight while riding on a train.
Matthau looked up at his wife with those big bloodhound eyes of his and said,
“You just ruined my trip to Dachau.”
The gag is greatly expanded in the sequel, A Tuna Christmas, in which Didi frequently sings Christmas carols to herself: Jingle bells, jingle… [drag]… [savor]… [exhale]… bells." It’s been years since I saw the play, but it has to be at least 5 or 6 times, each one of which got a huge laugh. I actually saw it before Greater Tuna, so when I later saw the original, I was disappointed that the joke only landed once.
–Cliffy
Shoeless:
This sounds very disturbingly like the “Gary vomiting” scene in Team America: World Police. Of course, a big part of that movie was lampooning the cliches of other movies, so it wouldn’t surprise me if that was intentional.
That’s a very old joke.
and the proper punch-line is “… I make a living” said in a kind of self-effacing yiddish borscht-belt voice
Percy:OK, let’s practice. (He gets inside box) Edmund comes in and says “Hullo Baldrick. You haven’t seen Percy have you?” And you say…
Baldrick: “No, my lord. I haven’t seen him all day.”
Percy: Brilliant!
Percy shuts lid on box. Blackadder enters.
Blackadder: Oh hullo Balders. Where the hell’s that cretin Percy? You haven’t seen him have you?
*Long pause. A look of intense concentration crosses Baldrick’s face as he tries to remember. Something he was to say… *
Baldrick: Yes, my lord. He’s hiding in the box.
Like all great comedic pauses; it doesn’t really work written down. It’s all in the timing and delivery.
Works perfectly, but only because reading “Yes, my lord” immediately conjours up Baldrick’s voice 
The best I can come up with is from the movie “Witness”. John Book (Harrison Ford) is trying to milk a cow.
Movie audience laughs. Slight pause. THEN the Amish guy laughs. Movie audience laughs again, louder.
Makes a note to resurrect this thread in ten years, with the post, “This one.”
Jackie Mason on that infamous Ed Sullivan Show. He made a long pause during his planned routine, and noticed Ed off-stage giving him a cue with two fingers up. Jackie stopped his routine and went off with his impromptu ‘Ed is giving me the finger’ schtick, giving everyone the finger.
One comedian (or should I say comedienne) I greatly admire for her great timing in word intonation and effect-increasing pauses is Wendy Liebman
There are lots of examples, but I really can’t think of many right off the bat.
But one that comes to mind is:
Curses! That was mine! MINE I tell you!
Not directly in a comedy, but the pause when Indiana Jones is about to grab his bullwhip but instead shoots.
At the end of “Some Like It Hot”
Jerry: You don’t understand, Osgood! Aaah… I’m a man!
…pause…
Osgood: Well, nobody’s perfect.
Another from Young Frankenstein is Terri Garr’s:
“… put… ze candle… back!”
Great Boo’s up, Edmond!
I can’t remember if I heard this retold or saw the clip. Johnny Carson and Buddy Hackett on the Tonight Show.
Buddy: So, I recently learned the secret of comedy.
(pause)
Buddy: Now you gotta ask me what the secret of comedy is.
(pause, famous Johnny look)
Johnny: So, Buddy, what’s the se-
Buddy: (leaning forward in his seat) TIMING!