Best use of a song in a film

[Nebuchadnezzar]

The Wanderers
Platoon
The Big Chill

There are a bunch

The first Batman movie with the Prince Soundtrack. Prince just had those songs laying around the house, asked what kind of scenes and put the soundtrack together in little time. Prince was a genius!

Good Morning Vietnam

A good soundtrack that I heard was on a Call of Duty video game, I don't remember which but it had "Sympothy For The Devil" on it.

One song that should have been on a soundtrack and it may have been written for a movie but never used is "I Write A Song" by Earth, Wind and Fire.

Saturday Night Fever

There are a bunch and these are just off the top of my head.