Well, let me do the math. Here is a list of the original muppet movies and their original songs.
TMM
- The Rainbow Connection
- Moving Right Along
- Can You Picture That
- Never Before Never Again
- I Hope That Something Better Comes Along
- I'm Going to Go Back There Someday
- The Magic Store
GMC
- Hey A Movie
- Happiness Hotel
- Stepping Out With A Star
- Night Life
- The First Time It Happens
- Why Couldn't We Ride
- Piggy's Fantasy
MTM
- Together Again
- You Can't Take No For An Answer
- Saying Goodbye
- I'm Going To Always Love You
- Right Where I Belong
- Somebody's Getting Married
- He'll Make Me Happy
I just think the first three feel most like musicals. MTM keeps a lot of its songs at the end, so I just do not feel the musical vibe, and GMC stops the songs after Piggy's Fantasy, not counting the "Hey A Movie" reprise. So, I think that TMM is the most musical muppet film, not only is it the first one, but the songs are scattered well, and they all work.