This is actually a pretty tough one, since for me the 70 are experienced only through other people's experiences. So here's what I got for you.
So here's why. The 70's were a fantastic time for music. Many of the elder statesmen of music, all genres, were in their heyday during that time. There was a lot going on. Disco, while not my favorite type of music, could not be ignored during this decade.
Pinball Wizard encapsulates much of the feeling of the 70's. The style and theatrics are a nod to the disco culture, but The Who can only be defined as a rock band. Even Elton John, who's performance is the most iconic, was known at the time as a rock musician, rather than the glam pop persona he is today.
It is not the best Who song, nor is it my favorite song of the decade, but I think it represents the decade quite well.