Favorite Song Written Within A Sitcom

What's your favorite song that was written within a sitcom's world, either by a character in the show or as a musical number (like in the Simpsons)?

I have so many, but I don't want to steal anyone's answers, so I'll just name two:

Bye, Bye Little Sebastian - Parks and Rec

"If you see something, say something. Come on and party tonight" - CJ, B99

EDIT: Please also name the show to give more context.