The most infectious tune ever written was apparently revisited in the new Muppet Movie (which I have yet to see) but I recall it best from the golden age of Sesame Street:
Slate tracked down the original song to, of all places, a 1968 Italian soft-core movie called Sweden: Heaven and Hell by Luigi Scattini.
From Amazon: "Edmund Purdom narrates a pseudo-documentary about sexuality in Sweden. It shows contraceptives for teen girls, lesbian nightclubs, wife swapping, porno movies, biker gangs, and Walpurgis Night celebrations. It also examines Sweden's purported drug, drinking and suicide problems. It features the original appearance of the Piero Umiliani's nonsense song 'Mah Nà Mah Nà' which was later popularized by 'Sesame Street' and 'The Muppet Show'"
Piero Umiliani, according to Wikipedia, "composed the scores for many exploitation films in the 1960s and 1970s, covering genres such as spaghetti western, Eurospy, Giallo, and soft sex films. Although not as widely regarded as, for example, Ennio Morricone or Riz Ortolani, he helped form the style of the typical European 1960s and 1970s jazz influenced film soundtrack, that later experienced a revival in films such as Kill Bill, and "Crepuscolo Sul Mare" in Ocean's Twelve."
In other words, he may be the most awesome composer you've never heard of. But Henson heard it when it briefly charted on US radio, and gave it eternal life.
Here's the original clip. It is pretty uncontroversial.