Greg Cox wrote:
"We'll Meet Again" at the end of Dr. Strangelove is pretty unforgettable, too.
A couple of times when I've seen that movie in a theater, the audience started singing along in an old-timey-picture-show sort of way. Kind of surprised me the first time.
I forgot to add before: Wicker Man
is a great choice, but I wouldn't object if someone actually classed it as a musical. A very eccentric one, to be sure...