Here I come, firstly I'd like to say that it's a real good question and the one nobody would neglect. I know most would simply tag it as a completely subjective topic and wouldn't even bother to try to justify their own statements on it but it's interesting to me and I always try.
1. Test of Time (Relevancy Upon Nowadays), it's useless to mention how popular song was at the time of its release, if it even was in the first place, but how well has it stood the test of time gotta be the most notable feature. And how would you determine it? The answer is always the people. If you're a big company in music, it's still the people who decide what is going to blast and what not. So if the people are still making covers for the tune, still talking about it, still adding it to their playlist, the song hasn't just survived, it lived and prospered through years, decades and only grew with lores and myths to it. This song is of a high quality.
2. Songcraft (Hard to Compose, Easy to Get Into), this is more about the composition itself, the technical side. They always say the genuis is in simplicity, and it more often than not, is the case. If you look into the structure of most popular songs, there is nothing really complex, it always has a catchy melody, positiveness in the lyrics and a beautiful arrangement. And while it may seem to be easy to do, it's deceiving because making simple tunes that overthrow progressive giants is far more harder than just following the pattern of whatever style you're into. There is a pattern in every subgenre but the standout is how well it is crafted and how good the parts are. Only the songs from talented and committed songwriters lived to see the days way after their release. There is no shame in sticking with such songs. It's mostly the songs that are made for the general public, that have the most attachment from people, they live and prosper because they're living on people's minds. They're of a high quality.
3. Lyrics (Message in the Text, Play of Words), another important topic that's so often neglected. I always considered myself a good wordsmith, because writing good lyrics has its own, special charm. It may be the last thing people pay attention to listening to music mostly on the background of doing something else. It's understood but if you take your time and lean back for a fresh new tune you've found, the quality of written lyrics will have a different impact on you. Depending on the subject and how good the wordsmith is, the lyrics alone might make you want to come back to this song after years and years it was, it can influence you to become an artist, a writer. It can fill your day with gladness and sunshine and make you a better person altogether. Because a good message and positiveness in everyday things make wonders. On the other hand, if the lyrics are of a very low quality and had been gathering all the filth possible in today's time, no wonder you'll be blue and depressed, it all has connection and a very clear affection. Good lyrics in good songs make those immortal and unforgettable. Being aroused by a quality lyrics is telling a lot about you in a good way.
4. Box Office (Selling, Playing, Portraying), the easiest one to get. How many copies did one sell throughout the time? How much people's attention it gathered? Did it impact the music industry in general? Was it seen, was it heard in films and other projects? How good it made at what it's being made for? If such info is within the reach, which is often the case, you can ground your reasoning upon it and build on it a mean argumentation.
5. Aftermath (Influence, impact), some standalone songs may give birth to a whole new direction in music as well as influence those who will eventually do that in the end. If you're aware that the song in question has achieved it once or twice, if it's impacted the pop music or let other substyles be, you can confidently say that this song is much more than just a one night stand and the music wouldn't be as complete without. It is no shame picking this one but instead, rather a necessity.