There aren't too many films where I can recall an original song being a big part of my enjoyment of the film. The music that tends to make an impact with me is incidental music scored for the entire film such as Jon Brion's "Punch Drunk Love", Thomas Newman's "American Beauty" or Yann Tiersen's "Amelie" to name 3 recent examples. Songs that register with me tend to be older tracks that have been used in scenes where they make perfect sense as opposed to being tacked on to add some extra value to the CD that accompanies the film. So with that in mind some of my favourite movie/song moments are:

"Son of a Preacher Man" - Pulp Fiction
"Stuck in the Middle With You" - Reservoir Dogs
"Lust for Life" - Trainspotting
"Sing Sing Sing" - Bright Young Things
"Moon River" - Breakfast at Tiffany's"
"Everything You Do is a Balloon" - Morvern Callar


And one that still resonates to this day "Man in Motion" from St. Elmo's Fire.