Wow. Nothing like starting off with a toughie. How do you pick one? One of my students asked me what my favorite movie was and I replied that things like that change as you get older...it is harder to have absolutes. I have a favorite comedy, a favorite action film, and so on. She looked at me like I was nuts and then proclaimed "Grown Ups" to be the best movie of all time.
Anyway, I might have to put "Band On The Run" by Paul McCartney as my favorite song. There is something to it that I have always liked. I grew up a Beatles fan, but gravitated towards Paul's solo stuff. I am fairly confident he will show up again on the list.