I guess trying to answer it starts with figuring out what qualifies as a song — and what "all time" means. First off, I think you'd almost have to say to qualify as the best of all time, you'd want something that had remained well known for two or three generations. Some of the best songs of all time may have been written in the past 25 years, but, really, we won't know which ones they are until we see if our grandchildren love them.
If you're looking at just the last 100 years or so, you'd probably have to include "Yesterday" on any short list. It's known worldwide, it's been recorded hundreds of times, and it's well known today by the kids and grandkids of the people who first loved it. spotify:track:3BQHpFgAp4l80e1XslIjNI:small
But in terms of "all time," 52 years ("Yesterday" came out in 1965) is a relatively short time. "Amazing Grace," on the other hand, has been around since 1779 and continues to be sung by millions of people every week, more than 230 years after its introduction. spotify:track:0DfcVxxGAkKipLcDYVE39h:small
If your idea of a song doesn't require lyrics, maybe you go back to a writer whose music changed everything that followed him — someone like Johann Sebastian Bach. You could make a case for any number of his works, including this one: spotify:track:7hzXlXMFx2UC4YWeIZqW1I:small
Or maybe you go back farther still, to the ancient folk songs. Take "Barbara Allen," for instance: That song was about 400 years old by the time Joan Baez got hold of it:
You hear entire histories of music in that song — in the melodic and harmonic structures, in the lyrical themes. I'm sure other countries and languages have songs that have stood the test of time the same way.
Now that streaming is hear to stay, honestly, it's virtually impossible to answer this question. We are continuously exposed to new music. There are just far too many that I love, so I"ll just pic the first song I like the most in my album list....