"My Love", by Paul McCartney & Wings. When I was nine years old, and I only knew this as a song on the radio, I thought that it was the greatest song I had ever heard. It extracted from me emotions I never knew I had.
In 2010, it tells me two other things: first, how much of McCartney there must be in "Abbey Road"; and second, that when McCartney gets it right, it can't be gotten any more right.