There is no best song. We all have our own individual tastes.
I have several favourites all falling between the years 1954 and 1983. Far too many to list, but I can tell you that "Bohemian Rhapsody" (which I find OTT and pretentious) by Queen (who I consider to be incredibly overrated) is not one of them.
I am also very appreciative of many of the songs from the first half of the century by the great American songwriters like Cole Porter, Rodgers and Hart, Jerome Kern and George Gershwin.
As for songs from 1984 to 1999, not for me I'm afraid, and don't get me started on 21st Century music!