A very personal tune for me, "The Air that I Breathe" is my song choice for when I need to sing loudly while driving in the car and have a cry.
I don't cry often, but a friend once told me that in order to be emotionally healthy you should pick a day in the month and cry your eyes out. I try and do this, sometimes not intentionally, but almost every time this song starts the tears flowing.
On a side note, the Hollies lead singers hair make me giggle a bit since my mom had the same hair style at one time! Sorry Mom:)