I guess I'm with LazyDolphin in the older than dirt category. There hasn't been one particular song that's changed my musical world but when I was about 12 or 13(in the early 80's) my brother got me hooked on Van Halen and Ozzy and then some new kids moved into my neighborhood with all the KISS albums that had been released at the time. I went through much of the 80's listening to the aforementioned artists and Motley Crue. Poison, Warrant, Cinderella, etc. Then one day one of my friends from school popped in Metallica's Master of Puppets. After about 30 seconds I said "Turn that (expletive) off!" A couple of days later he put it in again and after a couple of minutes I said "Let's listen to something else." The next time he put it in I said "Can I borrow that?" Soon after I was regularly listening to Metallica, Megadeth, and Queensryche, who went on to become my favorite band.
I don't follow the music scene very closely these days but I got hooked on Michelle Branch when she performed "Goodbye to You" on Buffy and back in March I bought the Evanescence cd
Fallen and I listen to it constantly. It's been several years since I heard something I like that much.