Could you clarify what you mean by pop music? The dichotomy in your question doesn't really make a lot of sense, as far as I can tell. Do you care about things that are popular (in the classical world today) now that were also smash hits in their day? Or do you care more about things that we don't care about now that were highly regarded in their day? I'm trying to figure out exactly what sort of historical phenomenon you are trying to get at...

I will say that it is not generally the job of a decent historian to answer the second question. Historians don't have the ability to judge what are "truly the best pieces," in fact, I'd wager that a good many historians here, myself included, would be very hesitant to say that there is such a thing as "truly the best." What there are, instead, are pieces of music that are good relative to some value system, but that sorting out which value system is "right" is an extremely fraught project.