IMO, both. I don't think it's the best song they ever did, it's not even the best song on the album! I think overrated implies that the work is not as good as critics/fans say it is, and it has taken on a life of its own, either through popular culture or otherwise. To me, the song fits the bill here. STH has become this anthemic piece and people have latched on to it in ways they more than likely haven't with other songs, and that's great - people will feel however they want about any given piece of music.
As for being overplayed, you could say the same about a lot of "classic rock" radio staples, such as Hotel California, Freebird, Money, Do You Feel Like I Do [basically anything by Peter Frampton] and Layla. They're all great songs don't get me wrong, but rock radio has driven them into the ground by playing them over and over and over and over again. I get bored of them easily because the joy and surprise of hearing them on the radio is completely gone.