Since there is no way I could ever limit it to just one I'll list my Top 10 favs(in no peticular order)
-Gnomes-the debut of the underpant gnomes, this episode took place way back in season 2, the moment that really stands out in my mind(other than the underpant gnome theme song) is when the finally see the underpant gnomes, and Cartman hits one them with a stick and the gnome calls him a pussy
-Something You Do with Your Finger-they start a boy band named Fingerbang, need I say more
-Do The Handicap Go To Hell?(part1)/Probably(part 2)-to sum it up Cartman becomes an evagelist, and it's revealed that Mormans got it right and are the only religion that goes to heaven, everyone else goes to hell
-Cripple Fight- Timmy vs. Jimmy, pretty much says it all right there don't it?
-Scott Tenorman Must Die-Cartman buys pubes, Cartman trys and trys to return pubes, but fails, then swears revenge, his masterplan... to train a pony to bite Scott Tenormans weiner off, in the end Cartman gets the ultimate revenge when he turns his nemisis parents in to chili and feeds it to him
-Towlie-this episode is centered around a talking towl that likes to get high, how can laugh watching this one
-Red Sleigh Down-Cartman(in atempt to score a big "nice") teams with Santa, Mr.Hankey, and Jesus to bring Christmas to Iraq, the best part of this episode is watch Jesus gunning down the evil Iraqi terroists
-Crips And Bloods- Timmy and Jimmy try to bring the Crips and Bloods together, with a lock-in
-Good Times With Weapons-the boys get ninja weapons and butters gets a nija star in the eye, the best part is when Cartman tries to use his power of invisability to get past of the parents and everyone else in town(to use his power he must take all his cloths off, thus exposeing himself to everyone in town)
-Woodland Critter Christmas-I reallyu don't know what to say here, but if you havn't seen this episode your really missing out, the woodland creatures having a blood orgy stands out as one of the funniest moments
those are my top ten