Well, it isn’t a sitcom, but I often say: Mr. McGee, don’t make me angry…
Homer Simpson: He did it because he’s stupid. That’s the only reason anybody does anything.
I’ve gotton a lot of mileage from that idea.
When something unfortunate happens to someone, like they stub their toe or burn their finger on a piece of toast:
“That’s a shame”
Also from Seinfeld:
“I’m definitely down for some rock climbing” (insert other fun activity)
I also say
“Crescent Fresh!!” from Sifl and Olly frequently
You wouldn’t like me when I’m angry!
I just used this one this morning!
There’s also “God’ll get you for that Arthur” from Maude
Appropriate and timely.
I just used it yesterday. I was drinking beer on my buddy’s porch and his puppy just would not behave.
“Screw you guys, I’m going home.” I can just about do Cartman’s voice.
“D’oh!” and “Eeexcellent” are such a part of my vocabulary that they didn’t occur to me as catchphrases until I read through this thread.
Hmm, others…
“Relaaaax, guy!” in Sadam Hussein’s voice from South Park.
“Good idea, bad idea,” and “Same thing we do every night, Pinky” from Animaniacs. I’ve also been known to randomly burst into “The Mr. Skullhead Show” song. “It’s the Mr. Skullhead Show! Starring him, Mr. Skullhead! That is the name of the show that he has, Mr. Skullhead!”
Wife and Spawn are at the high school’s modern setting of “A Midsummer Night’s Dream.” For reasons that I wish were obvious, I am not.
For reasons that I wish weren’t obvious but are, I’m singing the theme song from “Super Chicken.”
Nip it in the bud. - Barney Fife The Andy Griffith Show
Correctomundo - The Fonz Happy Days
Help me! Please help me! - Late Night With David Letterman
It’s always something! - Rosanna Rosannadana SNL
Isn’t that conveeeeenient! - The Church Lady SNL
“Maybe there’ll be cake” - Dollhouse (optimism)
“What the Frak!” - BSG (stupid drivers)
“It must be bunnies (or maybe midgets)” - Buffy (actual bunnies, or scapegoating)
“You’re a wee little puppet man!” - Angel (mock mocking)
“Noooooooooo” - specifically like Sheldon, BBT (you’re wrong on a fundamental level)
“No signs of sexual trauma” - CSI (dead bodies, dating, status report)
“And now for something completely different” / “No one expects the Spanish inquisition” “Nooo! Not the comfy chair!” - Monty Python (change of pace or mocking sameishness / surprise / mocking comfort)
“What a crazy random happenstance” - Dr Horrible (mock coincidence)
“Be awesome” - Chuck (peptalk)
“Engage” (with finger gesture) - ST:TNG (dude the light turned green)
“Resistance if futile” or “You must comply” / “You will be assimilated” - ST:TNG (do what I say / you cannot decline our invitation)
“My god it’s full of stars” - 2001 (shiny/pretty)
“It’s life sir but not as we know it” - Trek (weirdos)
“Enough!” - Futurama (this conversation has gotten too silly)
“There are no raisons!” - Futurama corruption of when the brain leave for no “raison” (why)
“It’s alive!” - Frankenstein (wow you managed to drag your sorry ass out of bed, or, I think there’s mold growing on that old pizza)
“These aren’t the droids you’re looking for” - Star Wars (ignore that)
“Your Jedi mind tricks won’t work on me boy!” - SW ROTJ (can’t fool me)
“That’s not even wrong” or “Mu” - not sure (your statement or question doesn’t make sense do to it’s underlying assumptions and therefore cannot be addressed at face value"
oops I guess those aren’t all sitcoms 
ah yes and “past nastification” / “nasty in the pasty” - Futurama (sexual history)
“yeah I want my cheesy poofs!” - South Park (greed / requests)
I haven’t, in any serious context, admitted that anything is better than the second biggest [or insert other superlative here] thing I’ve ever seen in my life since about 1976.
Whenever someone brings up giving to charity, I like to bring up my own: The Human Fund. Money for People.
Would you like some huh-whipped cream…? A little Miracle huh-Whip…?
Simpsons:
Excellent! - Montgomery Burns
South Park:
Well, that does sound pretty good. - Eric Cartman
Seinfeld:
Serenity now! - Frank Castanza
I use a lot of the Seinfeld ones, too. “SERENITY NOW!” and “I don’t wanna be a pirate!”
But, sadly, the one I use most often is from Home Improvement. “I don’t think so, Tim.”
(And then, if someone points out that their name isn’t Tim, I go into my “well if I don’t know your name I just call you Chief or Bub or ARE YOU FUCKIN’ WITH ME!!!???”
I’m a sad, strange little man.
When Mr. K sends me into a restaurant to see how long the wait is, I always come out and say, “You know…10…15 minutes. Cartwright!!”
My husband’s nickname is Willy. So when he tells me something kind of unbelievable, I always say, “Whachyootalkin’'bout,Willis?”
Whoa, no Arrested Development yet?
<singsong voice> “Hey Hermano.” --Buster
“I’ve made a huge mistake.” --GOB
“I don’t understand the question and I won’t respond to it.” --Lucille 1.
And so many more.
From the Simpsons–“Mistah Spahkle! For your happy washuuuuuuuu!!!”
Whenever I walk into the bathroom at work right after they’ve cleaned it, a Homer Simpson voice in my head says “Mmmm… urinal fresh!”