What are some examples of the best use of an existing song in a movie?
So songs that have been specifically made for the movie don't count.
A few examples that I can think of:
Back to the Future - Johnny B. Goode (cover)
Baby Driver - Hocus Pocus, Focus
Children of Men - King Crimson, The Court of the Crimson King
Children of Men - Radiohead, Life in a Glasshouse
Ferris Bueller's Day Off - The Beatles, Twist & Shout
Kill Bill - Tomoyasu Hotei, Battle Without Honor or Humanity
Pulp Fiction - Chuck Berry, You Never Can Tell
Pulp Fiction - Girl You'll Be a Woman Soon (cover)
Scott Pilgrim - Frank Black, I Heard Ramona Sing
Shaun of the Dead - Queen, Don't Stop Me Now
The Breakfast Club - Simple Minds, Don't You (Forget About Me)
What I notice is that I have 3 instances of Tarantino & Edgar Wright in this list. They're both masters with picking good songs for their movies. Another recent example I can think of is Immigrant Song in Thor: Ragnarok. And two instrumental tracks or melodies that were around before the movie:
2001: A Space Odyssey - Richard Strauss, Thus Spoke Zarathustra
The Exorcist - Mike Oldfield, Tubular Bells