Here are the two tracks I dropped when I decided to make it easier to keep my initial list down to a dozen by limiting it to selections before 1970:
God Save the Queen - Sex Pistols
While Anarchy in the U.K.
has the distinction of being the Sex Pistols first single in late 1976, God Save the Queen
from early 1977 was far more successful in offending the straights and was as a result much more commercially successful. It was the Sex Pistols highest charting single reaching #2 and catapulted the Sex Pistols to international stardom. It was therefore seminal in turning the punk movement into an international phenomenon.
Smells Like Teen Spirit - Nirvana
For better or worse, this was the release that launched grunge as a rock music genre.