"Kool Thing" by Sonic Youth (feat Chuck D from Public Enemy) was pretty much one of the first alternative/indie songs I ever heard.
I think this has stood me in pretty good stead for things that followed. Before I heard this tune it was all Cock Rock and thrash metal. After hearing this, and then Ride (on the same mixtape), and then Nirvana released 'Nevermind', things just got better and better.
What was the one tune that you heard (maybe during your formative years?) that forced you to throw everything else out the window and reconsider the musical sphere?