I'm not disagreeing with any of the fantastic candidates mentioned to date. But for me what makes Rock music more than just a form of contemporary music is that it's angry about something, The blues are meant to make you feel better, jazz is a journey through the vastly unexplored possibilities of musical theory, contemporary pop is a reflection of fashion and the current style. But Rock songs, great rock songs are about getting angry, either at the government or a girl, or your parents or whatever. Rock music is about breaking boundaries and expanding musical conventions, Hendrix distorting the classic tones of a fender guitar, Townsend smashing his guitar on stage, Joplin stretching a vocal to the edge of screaming rage. It's a release of emotion and not just the nice ones.
In that vein I will never ever forget certain moments in my life. The moment I was told that John Lennon was dead, the moment I found out that the police where breaking up, the first time I heard Sarah MacLachlan sing but most memorable for me was the first time I heard:
Killin in the name, by Rage Against The Machine.
it was a song that was as angry as I was, had all the screaming guitars that Rock music should have, introduced me to distorted wah-wah and was both simple and elegant at the same time. And if there's a lyric that has challenged authority any better than......well If you don't know the lyric I mean then you need to listen to this song, but I warn you there's some swearing in it.
So hat's off and to al the other songs on this list, and yes this is a silly loaded question that can never really be answered, but hey it's more about sharing our musical backgrounds and influences that anything else right ??