It is absolutely impossible to define the best music ever made, or the best song ever. Every kind of music conveys a range of different emotions to different people in different situations, so you can't define musical beauty, for the same reason you can't always say whether a paining is better than another one.
There are definitely some characteristics you can consider when you want to judge a song in terms of its "quality" (technique, lyrics, instruments, originality, etc.). I think I could give at least 100 examples right away of songs that I would associate with extreme high quality. I am going to give you just two examples, a classic one (not complete) and a curious one (a piano cover). Enjoy.