I have been attending theatre for more years than I would like to think about. There are two productions that are in the pantheon for me, and both opened 50 years ago appx. They are:
-- Darling of the Day...a delightful musical that had a score by Jule Styne and Yip Harburg. Patricia Routledge gave a performance that probably still ranks in my top 10 -15 performances in a musical by an actress. No advance, surprisingly good reviews, but Clive Barnes (NYT) trashed it...he was in a 'the future is rock' phase.
-- Drat! The Cat! Also had a delightful score and wonderful sets and costumes. Lesley Ann Warren and Elliott Gould(!) were the stars. He introduced the song 'She Touched Me'. The audience loved it, Walter Kerr loved it, its reviews were mixed; again, zero advance.
Others I would also mention:
-- Cyrano, with Christopher Plummer. He gave what is still a Top 5 performance by a male in a musical to me. I still listen to the score at least 5-6 times a year.
-- The Grand Tour, a Jerry Herman musical that had a typical Jerry Herman score (that is a complement). Joel Gray and Ron Holgate were both terrific. I remember them getting a rousing SO back in the days when SOs were RARE.
-- Maggie Flynn, with Jack Cassidy and Shirley Jones. I remember really enjoying it. It got mixed reviews, including some real raves, but never caught on.
-- Dear World. Another Jerry Herman musical, starring Angela Lansbury in a Tony winning performance. Was definitely flawed, had a stupid (but incredibly catchy) title song, but had a number of beautiful scenes and some terrific music.
More Recently:
-- The Scottsboro Boys...terrifically entertaining AND socially relevant.
-- The Visit. This may have been too bizarre for a broad audience, but it really should have enjoyed a longer run than it did.
-- Shuffle Along. Terrifically entertaining. I even REALLY enjoyed the criticized second act. We will never know how long it would have run if Audra MacDonald had not gotten pregnant.
-- Pacific Overtures. I did not love this show; still
There are a few legendary flops that I felt deserved to be flops, although aspects were Tier 1.
-- Mack and Mabel. Great score. Great lead performances. Great choreography and 'bigness'. Bumber story for which Gower Champion couldn't come up with a concept to diminish its abject dreariness. Maybe it needed a Pulp Fiction approach, i.e., kill the heroine with a drug overdose half-way through the show, and get back to cheery stuff. As it was, despite all the excellent components, it was one of the most depressing evenings I have ever spent at a musical.
-- Merrily We Roll Along...you had to be there to understand just how horrible an experience it was. I saw the Maria Friedman version last year and LOVED it. I have always assumed the problem lied with Harold Prince, perhaps the greatest director of musicals (ever? / in the last 50 years?).
-- I know there are a few others whose runs I thought were criminally short...just can't remember them right now.