BoRhap is a great pick, but I'm going with Stairway. I disagree with the "20 years ago..." comment. My kid is 16, and just finished her sophomore year of high school. At a dance celebrating her freshman year (it's complicated; her old school went through 9th grade, so it was akin to a graduation dance) the final song was... "Stairway to Heaven". And again this year, she was at a dance and there was two hours of Flo Rida and Sia and all that jazz, and the final song? You got it: Stairway to Heaven.
I also don't think you can have this conversation without Smoke On The Water or Freebird. (And I'm assuming this is just "rock", because there are standards that far eclipse any we've mentioned: "Respect", "Hound Dog", What A Wonderful World".
Honorable mentions:
- Something
- Satisfaction
- Good Vibrations
- Jenny Jenny
- Born To Be Wild