There's just too many good ones to just pick one, it's almost impossible. You can pretty much bet on laughing at any episode you watch. My most favorites are the ones that involve Cartman a lot.
The one where he plays a cop- "Respect my authoritaaa!" 😂
The one where he dresses up like Hitler(actually I think that happened twice) 😂
The one where he takes on the persona of Robert E. Lee and leads the South to victory.- "Yours truly, General Cartman Lee." 😂
The one where he starts a christian rock band.- "I wanna get down on my knees and start pleasin Jesus, I wanna feel his salvation all over my face." 😆
Anyways you get the picture. Like I said before too numerous to just name one.