Comfortably Numb would make my top 3. But if I had to pick ONE...
Life is Beautiful by Sixx AM.
That song was extremely influential and inspirational while I was getting clean and sober. It's tattooed on my forearm and I still get goosebumps every time I hear it.
Funny you mention tattoos. I have some of the lyrics to Fade to Black tattooed on my stomach. It helped me in the same situation. When I saw Metallica in Atlanta last year they played it. Brought tears to my eyes!