Episode 807: "Goobacks". The people of South Park are angry that the immigrants from the future are willing to work for next to nothing, causing the more conservative townies to publicly voice their displeasure. An in-universe case, as well, as the phrase starts out as "They're taking our jobs", but through repeated use, degenerates into this. The episode "W.T.F." takes this even further.
"TIM-MAY!"
Ooh, lib-a-lah!
"You go to hell! You go to hell and you die!" *
Said by Mr. Hat in Episode 101 as a Big "NO!" response to Kyle's request to go to the bathroom.
Episode 615: "The Biggest Douche in the Universe", from a parody trailer of a Rob Schneider film, which consists of almost nothing but variations of the word "derp".
More generally, "derp" or "herp derp"; originally used by Trey Parker and Matt Stone to describe lame, lowbrow comedy (as in the Rob Schneider example), but has evolved on the Internet into a more general response to anything extraordinarily stupid or to statements of the obvious.
Episode 1010, "Miss Teacher Bangs a Boy". The police officers say this about Ike (a preschooler!) when it's learned that he's been involved in a Teacher/Student Romance.
Episode 912, "Trapped in the Closet". Used as part of a Take That when explaining the tenets of the Church, most of which are indeed accurate. Later reused in Episode 1001: "The Return of Chef", replacing the Church with the more lawyer-friendly "Super Adventure Club".
After the Guitar Hero parody episode, any Guitar Hero vid on Youtube that shows someone getting to one million points will be invariably showered with comments saying "YOU ARE FAGS!".
Expect Harley riders to be called fags as well.
And if you think any of these aren't funny, then you need to get the sand out of your vagina.*
Episode 502, "It Hits the Fan". Cartman thinks Kyle's being too uptight about people saying the word "shit" after hearing it on the TV show "Cop Drama".
This one might have predated that episode, actually.
Episode 607, "Simpsons Already Did It", when Butters-as-Professor-Chaos is constantly reminded that his schemes to rain misery upon the people of South Park sound very similar to old episodes of The Simpsons.
Whatever! Whatever! I do what I want!*
Cartman in "Freak Strike"
BEEFCAKE!*
Episode 102 "Weight Gain 2000"
BUT MOOOOOOOOM *
Usually spelled BET MEEEEEEEM OR BIT MIIIIIIIM, after Cartman's odd accent
I am not "Myaeam".*
Episode 1514, "The Poor Kid" where Cartman finally gets called out on his odd accent
My name... is not... "Keeyle".
Craaaaaaaaaaab people, craaaaaaaaaaab people, taste like crab, talk like people...*
Episode 708, "South Park is Gay". The marching chant of the Crab People, who disguised themselves as the cast of Queer Eye For The Straight Guy in order to make all of the males in South Park "metrosexual".
From the movie where Saddam Hussein possesses the computer used by the U.S. Military for their invasion of Canada, causing the general to exclaim the quote.
