I recently heard a .wav of Cartman singing “You guys, are my best friends, through thick and thin, we’ve always been together, we’re four of a kind…” etc. Now I thought I had seen every episode of South Park but for the life of me I can’t remember ever seeing this one. Which episode was it?
Spooky fish. That’s evil Cartman doing the singing, FYI. 
He sounds like Sunny from “Out of Sync” when he sings like that.
What Zoggie said: the episode with the pet shop built on an Indian burial ground, that opens a portal to an alternate reality where everyone has goatees and is evil, except for Cartman, who in the alternate reality is, of course, a nice guy. Also the episode with “Aunt Flo.”
The “evil” Cartman episode is my favorite post-first season.
It aired about 3-4 weeks ago so you just missed it. (But I got in on tape now. Happiness.)
“Here Kenny, hold the knife facing away from you. That way you don’t cut yourself.”
“You’re such a nice boy.”
And that white line down the middle when both Cartman’s are on screen at the same time. One hella funny joke.