The use of the "Missa Luba" in "If" is one of the best soundtracks ever, haunting, violent.
"A Hard Day's Night" has to be well up on the list, including the "I Should Have Known Better" scene in the railway car.
Helen Morgan's "Bill" in Showboat.
Still, the most effective musical moment of all for me is the beautiful healthy young man in Cabaret singing "Tomorrow Belongs to Me" out in the countryside. The camera pans down to reveal that he is a member of the Hitler Youth. The song captures so many reasons why Hitler was so appealing to Germans in the 1930's better than anything else I know. You are sucked in like everyone else for a few brief seconds. That is really using music for cinematic purposes. Brrrrr.
Yours, Peter T.