Never has a song been such a clear trend marker. There is music pre-Nirvana, and there is most post-Nirvana.
It's not the best song, nor is it the most popular, but there's no denying that it completely opened up a new genre, style, and lifestyle, all of which most modern music derives some influence.
Beatles, Rolling Stones, etc--those guys were more influential bands. But i we're just talking about one single song, it's Nirvana's SLTS.