I love THE MUSIC MAN and think it needs to join ROCKY HORROR (which is so far the only musical in the canon). The whole movie is worthy of consideration, but let me make my case for inclusion based on a single scene, the song "The Sadder But Wiser Girl" :
It's a damn catchy tune perfectly performed by Robert Preston.
It's got a wicked sense of humor. "I cheer, I rave for the virtue I'm too late to save...I hope, and I pray, for a Hester to win just one more 'A'"
It's a great character bit. Harold Hill is a lecherous con-man who's planning to seduce the only person in town who can reveal him as a fraud.
The staging is super-weird. Robert Preston and Buddy Hackett singing a song about women who love having sex to a ten year old girl in a barn??
The whole movie is terrific. It's got heart and humor, the music runs the gamut from bizarre to instantly iconic, and the plot is a perfect encapsulation of how marketing and politics work in a grand scale (invent a problem, sell a solution). Even if it overstays its welcome by 30 min (sorry in advance for the runtime, the last act can be a bit of a slog), I adore this movie.