I'd say Mary Poppins. The music was very obviously attempting to catch that lightning-in-a-bottle magic of the Sherman Brothers, and it didn't work. I personally feel that Mary Poppins Returns actually got closer to that magic. That being said, the technical effects were pretty great, and it'll always have a special place in my heart as my first Broadway musical, so even if it's the worst musical I've seen, it's definitely not the worst one in the world.
"I think that when a movie says it was 'based on a true story,' oh, it happened - just with uglier people." - Peanut Walker, Shucked