What group or artist holds the record for appearing on the most movie soundtracks?
I think it's Jimi Hendrix. At least according to this website, 'All Along The Watchtower' holds the record in appearing in most movies. If you also count the other great songs of Jimi that were used in movies, I bet he holds the record.
Also I would consider placing Felix Mendelssohn at #1 place because who would argue that the song that holds the record in appearing in most movies is the 'Wedding March'?