The Music Man (1962)
Chosen by me because it influenced perhaps the greatest episode of the Simpsons ever.
Overlong, with some of the worst singers in a musical I have ever seen (and I've seen, and loved, Romance and Cigarettes) - a young Ron (Ronny) Howard is here with a terrible lisp but he's not much worse than some of the actors around him.
None of the plot really makes sense (a con artist is scamming a town into buying lots of music equipment), we are told our 'music man' has no ability at all but of course this is a musical so he keeps breaking into song. He may be the worst con artist ever as he basically just does what he says he was going to do.
The songs are often cleverly constructed (one has a bunch of gossiping townsfolk resembling hens pecking at the ground) but not very catchy and often completely superfluous to the plot.