"Who's Got the Greatest Mom In The World?" is song sung by Eric and Liane Cartman in the Season Ten episode, "Tsst".
Background
School counselor Mr. Mackey calls Liane Cartman into his office because Eric Cartman handcuffed Billy Turner's ankle to the school flagpole, then gave him a hacksaw to free himself. While discussing her sons worsening violent behavior, she begins to cry. Cartman tries to lighten the mood by singing this song.
Lyrics
Cartman: You're a great mom.
Who's got the greatest mom in the world?
I dooo.
My mom is number one in my heart.
It's true!
Mrs. Cartman and Cartman: My mom's the best mom, better than your mom.
It's singing together in harmony.