Not really a "Sondheim show" since someone else writes the book but my first experience was "Gypsy" with Angela Lansbury at St Louis' Muny Opera in 1974. Bonnie Langford was Baby June and when I met her a few years ago she was still in touch with Ms Lansbury.
Oddly enough the next theater event for me in St Louis was "Follies" in 2016 with a fine cast ( Bradley Dean, Emily Skinner, etc ).
My other half worked for Steve ( plus Hal, Ruth & Flossie ) so had the chance to meet him at parties and wedding receptions but what would I have said?