I have fond memories of watching The Muppet Movie in a theater with my family. I guess I was nine, since it came out in 1979. Seeing a movie in a theater was a huge treat for us, but what I remember the most is my parents breaking into song once we got home. They started singing "Moving Right Along," which thereafter became a family joke -- we'd sing it at each other at odd moments, especially when we were trying to leave the house or when we wanted someone to move.
I've enjoyed all of the Muppet movies since, but the original was my favorite. When The Muppets came out in 2011, I didn't expect much, but the reviews I read were good, so I raised my expectations, probably a little too much. It had a few good moments (my kids love the part where Chris Cooper raps), but it didn't approach my memories of the original.
I recently watched the original again and confirmed that it's a classic. It has such a goofy sense of humor and the cameos are delightful, especially Steve Martin and Mel Brooks. And "Moving Right Along" is still a fine song that I like to sing at my kids every now and then.