Pretty much any Randy Marsh-focused episode is pure gold.
I think my personal favourite Randy episode is when he buys an old Blockbuster store and forces his family to work there on Halloween.
Some other favs of mine:
Make Love, Not Warcraft (World of Warcraft)
Guitar Queer-O (Guitar Hero)
You Have Zero Friends (Facebook/TRON)
Over Logging (The Internet Goes Down)
The City Part of Town (Gentrification and Cheeseball "Concept" Neighbourhoods)
Child Abduction is Not Funny (Over-Protective Parents and Shitty Wall Mongolians)
Raisins (Hooters)
Up the Down Steroid (Jimmy Juices and Cartman Fakes Being Handicapped to Compete in the Special Olympics)
The Return of the Fellowship of the Ring to the Two Towers (Lord of the Rings)
My Future Self 'n' Me (Parents Lie to Kids Rather than Talk About Drugs)
Christian Rock Hard (Cartman Creates a Fake Christian Rock Band and Napster/Downloading Music)
... there's so many though.
People who just say "it's so vulgar" as a reason to not like South Park haven't even watched it, I reckon, cuz most of the episodes show how dumbass parents can be so dumbass and the kids are the ones who are actually the adults of the show.
I really don't get the love for the Scott Tenorman episode, I can't stand it.
So much goodness in the movie, especially the whole Satan/Hussein love story....
"I don't trust anything that bleeds for 5 days and doesn't die."
Man, the whole theater went "ooooooooooooooh" after that one. So good.