Hmmmm let me think... If I was feeling lonely, which I am a lot of the time, it would either be "Nobody Knows Who I Am" as sung by Linda Eder, or "No One Is Alone" done by either Kim Crosby or Bernadette Peters.
I also love Meredith Brook's "Bitch." It's a very fitting song for a lot of people, I think. For a happy mood song, it might be "Soak Up the Sun" by Sheryl Crow or "I Wanna Go Out Tonight" from Rent.
Caroline