Either:
Hide and Seek by Imogen Heap
I'm Here from The Color Purple
or Wherever You Will Go by The Calling
Hide and Seek because some of the lyrics are so true to my life. It sounds like such a relationship song, but when I'm wondering why I'm taking a certain turn, or how far I've come...I'll always think of this song.
I'm Here because I am a self-conscious person. And when I need a boost of confidence, I'll just listen to the end of this song, which has some of the most important lyrics I've ever heard.
And Wherever You Will Go because it reminds me of my friends and family. I probably have better reasons than that, but I can't think of them now.