It just means they’re memorable — being the “best” is more questionable — a lot of good things never see high popularity outside of niche groups, and might be remembered in that niche but forgotten by the public at large; a lot of bad things are remembered, sometimes because they were bad, sometimes because they coincided with other events.

But in general, time is a pretty good filter for quality (you’ll miss some good stuff, but if you only look at the culturally remembered stuff, you’ve still probably got a lifetime of good media)