Well, that's debatable, considering I dove into music so fast during my early teen years, but I would have to say it was either "I Hate Everything About You" by Three Days Grace, "Numb" by Linkin Park, or "Same Direction" by Hoobastank. And I don't know. The first two have more to do with peer-to-peer relationships than anything. That's the main focus of those years, so I guess that's why I connected to them. As for "Same Direction," it was due to the fact that I saw a Kingdom Hearts AMV chronicling Riku & Sora's relationship throughout the series.
None of these are my favorite songs though. I don't really have a favorite, but one of the best tracks I ever listened to is "Can you feel that, Captain Compost?!" by My Heart to Joy. The song captures raw emotion at its best and paints a picture of the place I've lived the past 8 years, both metaphorically and literally.