"Rainbow Connection" is far and away my favourite, but "I'm Going To Go Back There Someday" is a strong second. As a touring musician, I have enjoyed playing both of those songs almost as much as I enjoy listening to them.
I don't think "Hey, A Movie" gets nearly the credit it deserves. It's a great opening song and a fun number overall, right down to Sweetums hollering, "IS THERE ANY WAY TO STOP IT?" in the last verse.
A very selfish part of me would love to see the next Muppet movie open with that song, even though I'd prefer that it be a fresh new film and not a Muppet nostalgia trip.
"This Frog," a Kermit number from Sesame Street, is also vastly underrated in the Muppet universe. I found it on YouTube a couple of years ago and it blew me away. It still brings me a great adrenaline shot on down days.
There are so many other great ones..."Bein' Green," "Where The River Meets The Sea," "Our World," "Just One Person," "Movin' Right Along," "I Hope That Something Better Comes Along," "The Rhyming Song" (Link: "There's no hot water in my hotel"), "You and I and George," "The Christmas Wish," "It's In Every One Of Us," "The First Time It Happens," "Saying Goodbye," "Love Led Us Here," "Everyone Matters," "I Wish I Could Be Santa Claus," "Pictures In My Head," "Life's A Happy Song," "Man or Muppet"...
Quality music has kept me coming back to the Muppets year after year and I'm so happy that tradition continues to this day.