For me it is actually 2 songs that changed my life.
Up until 1998, my senior year in High School, I only really listened to bands that were on the radio. I did still listen to the popular Metal bands like Metallica, Iron Maiden, Megadeth, etc. but I never went anywhere beyond that. Then one day I bought a copy of Brave Words & Bloody Knuckles magazine mainly because it was only $4 and it came with a free CD. There were a few bands on that CD that I would get into later like Witchery and Naglfar but The 2 songs from it that forever changed me were "Farewell" by Sentenced and "Cassandra" by Theatre Of Tragedy. After that day I bought every single issue of BW&BK until they stopped printing the magazine in 2008, those CD's introduced me to so many bands that I lost count.