The biggest in my experience of following the theatre. I saw most of these and enjoyed them quite a bit, in honesty, though they were disappointments compared to the advanced expectations.
On a Clear Day...in its day, had incredible buzz, charged 20% more than any other show opened, had columnists quoting clever lyrics before it even opened. Lasted as long as it did only because it had a huge advance
Dear World...was expected to be a monster hit. Limped into NYC from Boston. Ran about 4 months. Surprisingly, I loved 80% of it, but the Boston version was much better. Also, one of the worst title songs ever. Maybe a fun song out of context, but awful in the context.
Mack and Mabel...all the element to be a smash hit but the actual story, which was a real downer. Mabel dies of an overdose in a musical with Keystone cops???
Merrily We Roll Along...expectations were so high and the result was so awful that few initially realized how great the score was. Harold Prince's concept didn't work. The cast wore tee shirts explaining who they were, e.g., producer, and you couldn't see them past the fifth or sixth row.
Mr. President...Irving Berlin's last show...huge advance, high expectations. DOA. Only Nanette Fabray survived the critical bashing...and not a very good song in the show.
Ballroom...Michael Bennett's first show after A Chorus Line to make it to B'way.
Some shoes had large advances because of the stars, not the creators, so I don't see them in the same light: Jennie, Breakfast at Tiffany's / Holly Golightly, Illya Darling, Wildcat.
Some created high expectation, not because of the creators or the stars, and I don't count them very seriously either. This would include Baker Street, Smile (Marvin Hamlish gotta lot of mileage out of the A Chorus Line score, which was the weakest part of the show), Merlin.
Finally, some because they were big hits in London, including Starlight Express and Chess, and did not transfer well. I loved both in London and they didn't work here, the former because there was no theatre suitable for what could have been a fun show; the latter because Trevor Nunn took a flawed show and ruined it.