The Muppets have really excellent taste in music. Blondie tore through One Way Or Another on the show, and Johnny Cash also made a memorable appearance. The Man in Black is smiling throughout, he looks like he's about to burst out laughing. Then there was the appearance Joan Baez made, singing a song by The Band that just looked like a proper session.
But I'm going with the frog. Kermit's take on Once In A Lifetime graced The Muppets Tonight, which ran during the nineties. It had me in knots and now forms the basis for my so-so impression of Ms.Piggy's paramour. He seems to be freaked out by lines like 'this is not my beautiful wife' and 'well, how did I get here?' Or maybe I'm reading too much into it.
Can a talking frog have an existential crisis?