The Ballad of Tweek and Craig was sung by an unknown middle-aged Japanese businessman in the Season Nineteen episode, "Tweek x Craig".
Background
Tweek and Craig's relationship becomes so popular it reaches international fame. A Japanese businessman talk-sings (Sprechgesang) a ballad honoring their love in an Akihabara karaoke bar in Japan.
A montage plays as Tweek and Craig are shown coming to terms with the townsfolk's belief they are a couple and hold hands while walking down Main street. Various other couples are shown such as Gerald and Sheila, Randy and Sharon, Butters and Charlotte via Skype and Mayor McDaniel's and her deceased husband's picture.
Lyrics
Maybe we should go away.
Put cares aside for just a day.
The world we face is a-not so big.
Not if we are strong like Tweek and Craig.
本当に 魚が 美味しい！ (Hontou ni sakana ga oishii!) ["The fish is really delicious!"]
Let's go back to when it was new.
Weren't we such happy people too?
Life was simple, problems vague.
We had it all, a lot like Tweek and Craig.
And in the darkness we all seek
A perfect love like Craig and Tweek.
We must fight or love's a goner.
We must hold Tweek and Craig with the Highest honor!
頑張ろう! (Ganbarou!) ["Let's give it our all!"]
いらっしゃい！(Irasshai!) ["Welcome!"]
I hope you know when you are down
That you turned my life around.
Just like the quote from Shakespeare said
I'm your Tweek, and you're my Craig.